Double Berry Sheet Pan Sponge Cake
BY Brimly.August 23, 2026
Photo By Editors @ the Brimly Test Kitchen
Bake: 32 MinPREP: 25 minGluten-freeServes: 12 Now, Why This Recipe and What Makes It Special?
This one began, the way good cakes so often do, with another cake. I had just made a batch of chiffon sponge cupcakes, and they came out so tender and cloudlike that I could not stop thinking about them. I wanted to see that same lightness stretched into something bigger. For a little while I turned the idea over, a roll cake maybe, but nothing quite settled until I remembered what season we are in. We are deep in birthday month here, and I wanted to give you a cake for the occasion. Something for the person scrolling the blog, looking for the thing they will make for their own celebration. So I reached for a shape that felt easy and generous, and I landed on a square.
What you have here is a beautiful, delicate chiffon. It is light on flavor by design, soft and spongy and plush, a cake that gets along with almost any frosting you set on top of it. A plain buttercream would be lovely and never wrong. But I wanted to nudge it somewhere a touch more interesting, so I leaned on strawberry extract for a soft berry note rather than reaching for a fruit puree. A little vanilla, a little more strawberry, and the smell alone was divine. For the top, I chose a blackberry buttercream, which turned the sweetest deep pink, almost a bubblegum color, and tasted every bit as good as it looked. I will say this gently, because I learned it the hard way: go easy on the blackberries. Just a quarter cup is plenty. I wish I had stopped sooner my first time around.
And here is the part that still catches me by surprise. I will finish the photos, sit down to write the post, and realize all over again that this is a gluten-free cake. It never feels like a compromise, and that is the whole point of what we do here at Brimly. So much of this work is for the person who felt looked over, who searched and searched and never found a recipe that seemed to have them in mind. You count too. If you cannot do gluten, or gums, or soy, the way we cannot in our own kitchen, I want you to have the bakes that are usually so hard to find, made simple enough to pull off yourself. That is why this one felt special to me, and why I loved making it for you.
Lessons Learned
The blackberries taught me the biggest lesson, so let me save you the trouble. Blackberries are a deeply seeded fruit, and they turn overwhelming fast when you add too many. Even at a quarter cup, mash them well, then add them to your butter one tablespoon at a time. Mix, taste, look at the color, and only then decide if it wants more. Adding the berries incrementally is the difference between a frosting you love and one that fights you with seeds.
Here is the second thing, and it startled me the first time it happened. When the berries go in, the buttercream can seize, meaning the butter starts to separate from the liquid and the whole thing looks broken. It is normal, especially in a cooler kitchen, and it is completely fixable. Start with room-temperature berries, never cold, to give yourself the best odds. And if it seizes anyway, do not panic. Set your mixing bowl inside a larger bowl of warm water, off the heat, and stir briskly with a spatula for a minute or so. As the butter softens, everything comes back together, fluffy and smooth, as though nothing ever went wrong.
Two more, both small but worth holding onto. Line your pan with parchment, leaving an overhang on two sides. This sponge can stick, and a non-stick pan loses its magic with age, so the parchment gives you a clean lift right out of the pan with no drama. Keep your oven at 340°F, not 350. That ten degrees matters here. And please rest the batter before it goes in. That necessary pause lets the baking powder wake up and gives the flour time to drink in the moisture, which is what keeps the crumb even and soft.
How to Make It Your Own
Now, the fun part, because there is so much room to play here. Fold a handful of sprinkles into the batter and you have a cheerful confetti cake. Leave the frosting plain, which is honestly how I love it most, and let the chiffon speak for itself. Reach for a different topping entirely, a whipped chocolate ganache would be gorgeous, or skip the frosting altogether, because this cake is wonderful on its own. You can change the extract too, all vanilla for something classic and simple. And if you are feeling ambitious, bake the batter in rounds and build a layer cake. It is gluten-free, and it will still work beautifully. Follow the steps as written and have fun with it. I cannot wait to see what you do.
Pro Tip: When you fold the blackberries into your buttercream, never pour them in all at once. Add the mashed berries by the tablespoon, mixing and tasting between each one, checking both the flavor and the color as you go. It is the surest way to land exactly where you want and to keep those seeds from ever becoming too much.
Summary Notes:
This double berry sheet pan sponge cake is a light, tender, gum-free chiffon baked in an everyday square pan and crowned with a soft blackberry buttercream that turns the prettiest deep pink. Strawberry and vanilla keep the crumb gentle, while the blackberries bring color and a beautiful berry richness on top. With simple ingredients and a method that rewards a patient hand, it is a lovely, welcoming cake for a birthday or any day that deserves one.
Tried it? Let us know how it went in the comments below.
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Ingredients
Cake:
1 cup + 2 Tbsp gluten-free flour (no xanthan gum) [BUY]
2 tsp baking powder
6 extra-large eggs, separated
¾ cup granulated sugar
2 tsp strawberry extract
1 tsp vanilla extract
¼ tsp fine sea salt
½ cup heavy cream
5 Tbsp corn oil, or any neutral cooking oil
Blackberry Buttercream:
2 sticks (1 cup) unsalted butter, room temperature
1¼ cup powdered sugar
¼ cup blackberries, room temperature, mashed
¼ tsp salt
Recipe Steps
Step 1
Heat the oven to 340°F. Butter an 8x8 square pan, then line it with parchment, leaving an overhang on two sides so you can lift the cake out cleanly later.
Step 2
Separate the eggs, placing the yolks in one bowl and the whites in the bowl of a stand mixer.
Step 3
Fit the whisk attachment and whip the whites on medium-high, adding the granulated sugar in three additions as they climb. Stop at soft peaks.
Step 4
To the yolks, add the strawberry extract, vanilla extract, and salt, and whisk until smooth.
Step 5
Add ¼ cup of the heavy cream along with all of the oil, and whisk together until just combined. Do not over whisk.
Step 6
Add the gluten-free flour and baking powder and mix into a thick paste. It will look almost too stiff, and that is exactly right.
Step 7
Add the remaining ¼ cup of cream and mix until the paste loosens into something smooth and pourable.
Step 8
Fold about ½ cup of the whipped whites into the yolk base until no streaks remain. This first addition lightens the heavy base so the rest of the whites fold in gently.
Step 9
Pour the lightened base back into the bowl of whites and fold slowly, turning the bowl as you go, just until no white streaks remain and not a stroke longer.
Step 10
Let the batter rest 5 minutes. This lets the baking powder activate and gives the flour time to absorb the moisture.
Step 11
Pour the batter into the prepared pan, level the top, and bake at 340°F for 32 minutes, until the top springs back and a tester comes out clean. Lift the cake out by the parchment and cool completely before frosting.
Blackberry Buttercream:
Whip the room-temperature butter on medium-high until it turns pale and noticeably lighter, about 3 to 4 minutes. Add the powdered sugar in a few additions, mixing low at first so it does not fly, then whipping until fluffy. Add the salt. Now add the mashed blackberries one tablespoon at a time, mixing and tasting after each, watching the color deepen into that soft pink until it is exactly where you like it. If the buttercream seizes and looks separated, do not worry. Set the bowl inside a larger bowl of warm water, off the heat, and stir with a spatula for a minute until the butter softens and it comes back together, smooth and fluffy. Spread over the fully cooled cake, then slice into neat squares.
Storage:
This cake is best enjoyed fresh, while the crumb is at its most tender and the buttercream still soft. Cover leftovers tightly or move them to an airtight container. Because of the butter in the frosting, store in the refrigerator for up to 3 days, and bring the slices back to room temperature before serving so the buttercream softens and the sponge returns to its full, airy self.
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