Ingredients
Method
Step-by-Step
- Heat the oven to 350°F (175°C). Grease two 8-inch round cake pans, line the bottoms with parchment, and lightly flour the sides.
- Whisk the cake flour, baking powder, and fine salt together in a medium bowl.
- Beat the softened butter and granulated sugar for 3 to 4 minutes, until pale, smooth, and fluffy.
- Add the egg whites gradually, beating after each addition, then mix in the vanilla extract. Stir in the sour cream until the mixture is smooth.
- Add the dry mixture to the butter mixture in three additions, alternating with the milk. Mix only until the batter comes together, scraping the bowl as needed.
- Fold in the rainbow jimmies gently by hand. Divide the batter evenly between the prepared pans and bake immediately.
- Bake for 25 to 30 minutes, until the centers spring back lightly and a toothpick comes out with a few moist crumbs. Cool the cakes in their pans for about 10 minutes, then turn them onto a rack and cool completely.
- For the buttercream, beat the softened butter until smooth. Gradually add the sifted confectioners’ sugar, then add the cream or milk, vanilla, and salt.
- Beat the frosting for 3 to 5 minutes, until light and fluffy. Level the cake layers if necessary, place one layer on a serving plate, spread buttercream over it, add the second layer, and frost the top and sides.
- Finish the cake with extra rainbow sprinkles. Chill briefly if needed for cleaner slices, then serve at room temperature for the softest buttercream and crumb.
Notes
Use rainbow jimmies rather than nonpareils because jimmies hold their color and shape better. Keep the butter, egg whites, sour cream, and milk at room temperature, avoid overmixing after adding flour, and fold in the sprinkles at the very end. Store the finished cake covered at room temperature for a short serving period or refrigerate it for longer storage, bringing it back toward room temperature before serving.