Instructions

Make the crumb:

1. Heat the oven to 300°f.
2. Combine the sugars, flour, baking powder, salt, and sprinkles in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed until well combined.
3. Add the oil and vanilla and paddle again to distribute. the wet ingredients will act as glue to help the dry ingredients form small clusters; continue paddling until that happens.
4. Bake for 15 minutes at 300°f.
5. Let the crumbs cool completely before using in a recipe or scarfing by the handful. stored in an airtight container, the crumbs will keep fresh for 1 week at room temperature or 1 month in the fridge or freezer.

Make the cupcakes:

1. Heat the oven to 350°f.
2. Combine the butter, shortening, and sugars in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes. scrape down the sides of the bowl, add the eggs, and mix on medium-high for 2 to 3 minutes. scrape down the sides of the bowl once more.
3. On low speed, stream in the buttermilk, oil, and vanilla. increase the mixer speed to medium-high and paddle for 4 to 6 minutes, until the mixture is practically white, twice the size of your original fluffy butter-and-sugar mixture, and completely homogenous. don’t rush the process. you’re basically forcing too much liquid into an already fatty mixture that doesn’t want to make room for that liquid. there should be no streaks of fat or liquid. stop the mixer and scrape down the sides of the bowl.
4. On very low speed, add the cake flour, baking powder, salt, and the 50 g (¼ cup) rainbow sprinkles. mix for 45 to 60 seconds, just until your batter comes together. scrape down the sides of the bowl.
5. Stir in 1 cup of birthday cake crumb.
6. Line a cupcake pan with liners and fill each liner about 2/3 full of batter. Sprinkle remaining sprinkles over top.
7. Bake the cake for 30 to 35 minutes. the cupcakes will rise and puff, but will remain slightly buttery and dense. at 30 minutes, gently poke the edge of the cake with your finger: the cake should bounce back slightly and the center should no longer be jiggly. Leave the cupcakes in the oven for an extra 3 to 5 minutes if it doesn’t pass these tests.
8. Take the cupcakes out of the oven and cool on a wire rack or, in a pinch, in the fridge or freezer (don’t worry, it’s not cheating). Cupcakes should be completely cool before frosting.

Make the birthday cake buttercream:

1. Pulverize two cups of birthday cake crumb.
2. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium speed until soft, about 1 minute.
3. Add 2 cups of the confectioners’ sugar, the salt, and vanilla. Beat on low speed until the sugar is well incorporated.
4. Add another 2 cups sugar and the cream.
5. Add remaining cup of sugar as needed to thicken the frosting.
6. Add pulverized birthday cake crumb. Beat on medium speed until the frosting is smooth and fluffy, about 4 minutes.