When Amirah Kassem was in the throes of opening her New York City bakery, Flour Shop, she took a break one afternoon to bake a cake for a friend’s son’s birthday. She’d made plenty of cakes for friends in the past, but this time was different: After she baked a full set of bright rainbow cake layers, she cut a hole in the center of each and filled the inside of the cake with multicolored sprinkles. She imagined an explosion of sprinkles when the cake was cut — like an edible version of all the piñatas she’d seen at parties while growing up in Mexico. “When the birthday boy sliced it open, it was a total paparazzi moment,” Amirah says. “Everybody pulled out their phone to take a picture.” Realizing she had a hit on her hands, she added the creation to the Flour Shop menu, and the cake exploded again — on Instagram. Famous fans like Beyoncé, Katie Holmes and Kelly Ripa shared photos of it, and Instagram called out rainbow and unicorn cakes as a top trend, using a Flour Shop cake as an example. Amirah shared the recipe (from her book, The Power of Sprinkles) so you can make one yourself!
Preheat your oven to 350˚ and put the oven rack in the middle. If you are using a convection oven, set it to 325˚.
Combine the flour, baking powder and salt in a large bowl and whisk until they are really mixed together. Set aside.
In a separate bowl, use an electric mixer on medium speed to blend the butter and sugar together until they become fluffy. Make sure to scrape the sides of the bowl with a spatula so it’s all mixed in from the sides.
Add the eggs, one at a time, to the butter-sugar mixture, with the mixer on medium speed. Again, make sure to scrape the sides of the bowl. Add the vanilla to the milk and set it aside.
Mix about one-third of your dry ingredients into the butter mixture, then blend in half of the milk mixture, always mixing on medium speed. Mix in the second third of the dry ingredients, then the remaining milk. Scrape down the bowl, then mix in the last of the flour mixture. You don’t want any lumps, but don’t overmix it—stop the mixer as soon as the batter is smooth.
Divide the batter evenly among six small bowls. They don’t have to be exactly identical, but you want them to be close. (It’s about 1 cup of batter per bowl.)
Color the batter individually in rainbow colors: We use purple, blue, green, yellow, orange and pink for our six-layer cakes. Start with a tiny drop of food coloring, stir it in completely, then add more until it is your desired color. (The baked cake will come out pretty close to what you see.)
Spray six 6-inch round baking pans with cooking spray, then pour in the colored batter. (If you don’t have 6 pans, cool, wash, dry and respray your pans before you refill them with batter.) Bake the cakes two at a time for 8 minutes without opening the oven door, then rotate each pan and bake for another 8 minutes, or until a toothpick comes out clean when you insert it into the middle of the cake.
Let the cakes cool in the pans for 5 to 10 minutes, then flip them over onto a baking sheet or cooling rack and let them cool completely before you frost them.
To make ahead, bake the cake layers and let cool, then wrap individually in plastic wrap and refrigerate 2 to 3 days.