1-Preheat the oven to 350 degrees Fahrenheit. Prepare three 4-inch cake pans or one 6-inch cake pan by greasing with cooking spray and lining the bottom with wax or parchment paper.

2-In a mixer, cream the butter and sugar on medium-high speed until light and fluffy, about 3 minutes. Add the egg on low speed and mix until combined, scraping the bowl as needed. Add the vanilla extract and sour cream, then beat on high for one minute; the batter may look curdled but will smooth out.

3-On low speed, add the dry ingredients cake flour, baking powder, and salt all at once and mix until just combined. Slowly pour in the milk and mix on low for about 30 seconds until just combined. Scrape the bowl and whisk a few times to remove any lumps; the batter will be slightly thin.

4-Fold in the flour-coated rainbow sprinkles using a rubber spatula. Divide the batter evenly into the prepared pans. Bake for 18-22 minutes for 4-inch pans or 24-28 minutes for a 6-inch pan, or until the cake springs back when touched and a toothpick inserted comes out clean. Allow the cakes to cool completely before frosting.

5-For the buttercream, beat the butter on high speed for about 7 minutes until light and almost white. Add the powdered sugar in two parts on low speed, scraping the bowl each time. Add the vanilla extract, milk, and salt, then mix until smooth. Reserve 1/4 cup of the buttercream without sprinkles, and fold the sprinkles into the remaining buttercream.

6-To assemble, frost between the layers and around the cake. Chill the crumb-coated cake for at least 30 minutes. Use the reserved plain buttercream to pipe a star border on top and decorate with additional rainbow sprinkles.