1-Preheat oven and prepare pans: Preheat oven to 350°F, grease two 8- or 9-inch round cake pans, line bottoms with parchment, and grease parchment.

2-Cream butter, oil, and sugar: Beat softened butter, oil, and sugar on medium-high until pale and fluffy, about 3-5 minutes.

3-Add vanilla: Mix in clear vanilla extract until fully incorporated.

4-Whisk dry ingredients: Sift together cake flour, baking powder, and salt in a separate bowl.

5-Alternate dry and wet ingredients: On low speed, add flour mixture in three additions alternating with buttermilk in two, beginning and ending with flour; mix just until streaks disappear.

6-Whip egg whites: In a clean, grease-free bowl, whip egg whites to soft or medium peaks.

7-Fold in whipped egg whites: Gently fold whipped egg whites into batter by hand with a spatula, taking care not to deflate, until no white streaks remain.

8-Coat sprinkles with flour: Toss rainbow jimmies with 1 tablespoon flour until evenly coated.

9-Fold in sprinkles: Gently fold coated jimmies into batter until just distributed; do not overmix.

10-Fill pans and bake: Divide batter evenly between pans and bake 25-30 minutes until toothpick inserted in center comes out clean.

11-Cool the cake layers: Cool cakes in pans for 10 minutes, turn out onto wire racks, remove parchment, and cool completely (about 1 hour).

12-Make the buttercream frosting: Beat softened butter until creamy; gradually add powdered sugar, clear vanilla, and salt; add heavy cream 1 tablespoon at a time until smooth and spreadable.

13-Apply the crumb coat: Place one cooled layer on plate, spread a thin layer of frosting over top and sides, and refrigerate 30 minutes.

14-Apply final frosting layer: Generously spread remaining frosting over chilled cake with offset spatula, smoothing sides with bench scraper.

15-Decorate and serve: Scatter additional rainbow jimmies over top and sides just before serving.