Preheat oven to 350°F (180°C). Butter two 8-inch round cake pans and line with parchment. In a stand mixer, whisk flour, sugar, baking powder, and salt. Add butter and beat on medium for 2 minutes until mixture looks like wet sand.

Whisk buttermilk, egg whites, and vanilla extract together. With mixer on low, add the wet mixture to dry in three additions, then beat on medium for 30 seconds until smooth.

Fold in rainbow jimmies with a spatula. Divide batter between pans, smooth tops, and bake 25-30 minutes until a toothpick comes out with a few moist crumbs. Cool 10 minutes in pans, then turn out to cool completely.

For buttercream, whisk egg whites and sugar over simmering water until 160°F (71°C). Whip on high speed until stiff peaks form and bowl is cool, about 5-7 minutes. Gradually beat in butter, then vanilla bean paste, salt, and pink gel coloring until silky.

Place one cake layer on a plate, spread with 1 cup buttercream, add extra sprinkles if you like, and top with second layer. Apply a thin crumb coat, chill 20 minutes, then frost generously with remaining buttercream.

Dust the top with edible glitter, press sprinkles around the bottom edge if desired, and serve.