1
Preheat oven to 350°F (175°C).
2
In a large bowl, combine flour, baking powder, and salt. Whisk thoroughly to distribute leavening agents evenly.
3
In a stand mixer with paddle attachment, combine softened butter, granulated sugar, and vegetable oil. Mix on medium speed for about 3 minutes until light and fluffy.
4
Add egg whites in three additions, mixing well between each addition and scraping down the bowl as needed. Add vanilla extract last.
5
On low speed, alternately add flour mixture and buttermilk in three additions each. Finish mixing by hand to avoid overmixing, which would create a dense, gummy cake.
6
Fold rainbow jimmies into the batter.
7
Divide batter evenly among three greased 8-inch round pans lined with parchment rounds. Smooth the tops with an offset spatula. Optionally, use damp cake strips around the edges for even baking and flat layers.
8
Bake at 350°F (175°C) for 32–38 minutes until springy in the center, a skewer comes out clean, and edges pull slightly from pan. Rotate pans if needed for even baking.
9
Cool cake layers to room temperature.
10
In a stand mixer with paddle attachment, combine softened butter and salt. Mix on medium speed for about 2 minutes until smooth and creamy.
11
On low speed, gradually add powdered sugar and heavy cream as needed (1 tablespoon at a time) until frosting reaches desired consistency. Add vanilla extract. Increase to medium speed and beat 1–2 minutes until fluffy.
12
Place first cake layer on a serving plate. Spread a thin layer of frosting on top, add second layer, repeat, and top with third layer.
13
Spread frosting evenly over entire cake using an offset spatula. Smooth with a bench scraper. Optionally, press rainbow jimmies around the lower third of the cake.
14
Optional: Make pink drip. Combine 1 cup candy melts, ¼ cup white chocolate chips, and 3–6 tablespoons heavy cream in a bowl. Microwave in 20-second bursts at half power, stirring between bursts, until mostly smooth. Add pink gel food coloring. Cool to desired thickness, then pour over cake top and let drip naturally.
15
If using drip, refrigerate cake briefly to stop dripping.
16
Transfer remaining frosting into a piping bag fitted with a large open star tip. Pipe dollops around the top edge of the cake.
17
Serve immediately or refrigerate until ready to slice. Store covered at room temperature for 3 days, in the fridge for 5 days, or freeze well-wrapped for up to 3 months.
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