Preheat your oven to 375 degrees F. Line a 12-cup standard muffin tin with paper liners and set it aside.

In a large bowl, mix together the cake flour, baking powder, and kosher salt.

In another bowl, whisk the milk, egg whites, and vanilla extract together until well combined.

In the bowl of an electric mixer fitted with a paddle attachment, beat the unsalted butter and sugar on medium speed until the mixture is light and fluffy, roughly 2-3 minutes.

With the mixer on low speed, alternate adding the dry ingredients and the milk mixture in three parts, starting and ending with the dry ingredients. Beat just until everything is incorporated.

Gently fold in the rainbow jimmies using a rubber spatula until just moist.

Scoop the batter evenly into the lined muffin tray.

Bake in the oven for 15-18 minutes, or until a tester inserted into the center comes out clean.

Remove the cupcakes from the oven and let them cool completely on a wire rack before frosting.

In the electric mixer bowl, beat the unsalted butter and confectioners’ sugar on medium speed until light and fluffy, about 1-2 minutes.

Gradually add the heavy cream and vanilla extract while the mixer is on low speed, mixing just until combined.

Increase the speed to medium-high and beat until the frosting is light and fluffy, roughly 1-2 minutes.

Use a small offset spatula or a pastry bag with a decorative tip to frost the cupcakes.

Sprinkle with additional Fruity Pebbles if desired.