Ingredients
Method
Make the Cake
- Preheat the oven to 350℉ and grease a 9x13-inch baking pan. Line it with parchment paper for easy removal.
- In a large bowl, beat the 1 cup unsalted butter and 2 cups granulated sugar together using an electric mixer until light and fluffy, about 5 minutes. Beat in the 4 large eggs one at a time, then add the 1 tablespoon pure vanilla extract.
- In another large bowl, whisk together the 3 cups all purpose flour, 1 tablespoon baking powder, and ¼ teaspoon fine sea salt. With the mixer on low, add half the flour mixture to the butter and egg mixture and beat until just combined. Add the 1 ⅓ cups buttermilk and mix well to combine. Add the remaining flour mixture and beat until just combined, scraping down the bowl as necessary. Fold in the ¼ cup rainbow sprinkles.
- Pour batter into the prepared pan, place in the oven, and bake for about 45 minutes, until a tester inserted in the middle of the cake comes out clean. Remove from oven and place on a rack to cool completely in the pan.
Make the Frosting
- In a large bowl, cream the room temperature ¾ cup unsalted butter until fluffy. Beat in the 4 cups powdered sugar, then add the 1 teaspoon blue food coloring, beating until well incorporated. If the frosting seems a little dry, add 1-2 tablespoons milk, and beat until fluffy.
- Assembly: You may leave the cake in the pan or gently remove it (I kept it easy and left it in the pan). Frost the top of the cooled cake with the frosting, then liberally cover the top with decorative sprinkles. Cut the cake into squares to serve.
Notes
Tips
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- Shake the buttermilk. Buttermilk, like yogurt, needs a good shake or stir before using it. So, exercise those muscles and shake up the buttermilk before adding it to the batter.
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- Use room-temperature ingredients. Especially important in the frosting, but also for the cake: Room temperature ingredients, particularly the butter, will easily blend with the other ingredients.
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- For the best texture: If you've kept the cake in the fridge, bring it to room temperature on the counter before serving. I try to allow about 15-30 minutes for the cake to "warm up" after it's been in the fridge. You will not regret it!
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- Make it a chocolate chip cake. You can use the cake base to make chocolate chip unicorn bars! Instead of (or in addition to?) sprinkles, add mini chocolate chips to the cake batter
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- Different frosting colors. I love blue frosting but you really could use any color you like! Pink would be very pretty, or you could do a rainbow combination of different-colored buttercream.
- Counter - Cover the cake with foil and keep it on the counter for up to 3 days.
- Fridge - The cake, covered or in an airtight container, will keep well in the fridge for up to 1 week. Let it sit on the counter for about 30 minutes before serving if it's been in the fridge.
- Freezer - Store the unfrosted cake by wrapping it tightly in a layer of plastic then a layer of foil, and placing it in the freezer for up to 2 months. Thaw the cake in the fridge before frosting. Or place the assembled cake or pieces of cake in a freezer-safe container and store it in the freezer for up to 2 months. The texture of the frosting will not be as fluffy when it's thawed, but it will still taste good. Defrost the cake in the fridge before serving.