Chocolate Sheet Cake with Rainbow Petal Buttercream

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A super chocolatey, moist cake made solely out of pantry ingredients (egg-free and dairy free) topped with a basic(ally fantastic) two vanilla buttercream. The cake is decorated with spring-like buttercream petals, colored like a gentle, beautiful rainbow.

The artistic side of me has a deep appreciation for sheet cakes. I’ve talked about this before, but I used to find them so sub-par to pinterest-perfect layer cakes until I was at a kid’s birthday party two years ago. The hosts had served a grocery-store bought sheet cake and I noticed how easy it was to serve everyone a slice of cake (compared to how I struggled with my daughter’s first birthday pumpkin layer cake!). Last year I made one of my cherry blossom cakes in sheet-pan form and a few months later this sunshine cake. Beyond the practicality of serving a large party, there is the fact that any flavored cake can be covered in white buttercream which becomes a canvas for you to get very creative with. Plus, if you pick the right cake (like this one!) you can keep it and have a delicious and beautiful snack for days and days!


I’m usually a double chocolate kind of girl but this cake and buttercream are weak points for me. I’ve been making KAF’s original cake pan for years now, in various forms (like these cupcakes!). It is vegan, but you wouldn’t know better just by taste. My husband, a steak lover and skeptic of all foods vegan, has declared it his favorite cake.


If you’ve made any of my vanilla cakes before, you’ve likely made this two vanilla buttercream. I use the two because I think the depth of pure vanilla makes a delicious base and the imitation vanilla brings out a charmingly sweet taste, without overwhelming. If you use organic powdered sugar, you’ll be in for the absolute perfect vanilla buttercream.

rainbow petals

I had received a few requests for how to do this petal decoration after my 2020 cherry blossom cake (scroll to the bottom), so thought I’d do it again. It was also an excuse for me to do something I could get lost in for an hour (with a delicious outcome, obviously!) while I stowed away my worries during this pandemic. This made every afternoon enjoyable for days later. I hope it brightens your day and inspires your own meditative buttercream decorating!


Rainbow Petal Sheet Cake


  • The chocolate cake recipe is adapted, just barely, from King Arthur Flour’s Original Cake Pan Cake. I love this recipe as does everyone who has tried it when I’ve made it. I doubled it so it would nicely fill a sheet pan. The original recipe suggests vegetable oil but I often use canola or even olive; the latter makes a wonderful complement to the dutch cocoa.
  • This cake is easily halved to fit in an 8” round or square pan, or even quartered to fit into a loaf pan. To quarter : 3/4 cups all purpose flour, 1/2 cup sugar, 2 tablespoons dutch cocoa, 1/4 teaspoon salt, 1/4 teaspoon soda, 1 teaspoon vanilla, 1/2 tablespoon vinegar, 2 heaped tablespoons oil and 1/2 cup water.
  • Let’s talk about those pretty petals! I did this same design for my 2020 cherry blossom cake (pictured below) but with a different tool. Here’s what I used for this one: this artist’s spatula set (specifically for this cake the one on the far left) and, most aptly titled, an artist’s muffin style palate (rather than dirty a bunch of bowls!) The colors I used are the same ones I used for this tiered rainbow cake: Americolor Dusty Rose, Americolor Peach, Wilton Yellow, Americolor Laurel, Americolor Wedgewood, Americolor Violet, Chefmaster Purple.
  • I have found that when exposed to sunlight, food coloring can fade so do your best to avoid it!
  • This cake keeps moist in an airtight container for days.

Pantry Staple Chocolate Sheet Cake

  • 3 cups all-purpose flour
  • 2 cups granulated sugar
  • 1/2 cup dutch process cocoa
  • 1 teaspoon fine sea salt
  • 2 teaspoons baking soda
  • 1 tablespoon pure vanilla extract
  • 2 tablespoons vinegar
  • 2/3 cup canola oil or olive oil
  • 2 cups cool water

Vanilla Buttercream

  • 1 cup butter, softened
  • 3 cups organic powdered sugar
  • 1 teaspoon pure vanilla extract
  • 1 teaspoon imitation vanilla
  • 3/4 teaspoons fine sea salt
  • 1 tablespoon milk, buttermilk or cream
  • assorted food colorings


  1. Make your buttercream: beat the butter with the vanillas and salt on medium until very light and fluffy. Add the powdered sugar and beat to combine. Add the milk and stir to combine. Set aside.

  2. Preheat oven to 325 and line a sheet cake pan with a parchment paper sling.

  3. In a large bowl whisk together the dry ingredients. To the bowl add the water, oil, vanilla and vinegar. Whisk to combine.

  4. Pour into prepared pan and bake for about 30 minutes, checking the center with a toothpick for doneness.

  5. When cake is cool, cover with a layer of white buttercream to act as a background. Set the cake in the fridge for about 15-20 minutes so the buttercream will harden and the background won’t mess up as you decorate. Add 2-3 tablespoons buttercream to a small bowl and color with food coloring. Use the artists’ spatula to scoop up a bit of buttercream on the back and gently “wipe” it in a sliding motion onto the white buttercream. Try not to put too much buttercream, about 1/8 teaspoon per petal is enough. Repeat through whole cake.

  6. For an ombre effect or to transition more smoothly between complementary colors, use the same bowl to add a color and mix.

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