Preheat the oven to 350°F (177°C). Grease a 6-inch cake pan and add a circular piece of parchment paper to fit in the bottom. Also grease the "top" of the parchment paper once it's in the pan. Set aside.

Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on high speed until smooth and creamy, about 2 minutes. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add the egg whites and vanilla extract. Beat on medium-high speed until combined, then beat in the sour cream. Scrape down the sides and up the bottom of the bowl as needed. With the mixer on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, slowly pour in the milk until combined. Do not over-mix. Gently and slowly whisk in the sprinkles. The batter will be slightly thick.

Pour/spoon the batter into the pan. Bake for 26-28 minutes, or until a toothpick inserted in the center comes out clean. Allow the cake to cool completely before frosting.

Frost cooled cake with vanilla buttercream. I made a "half" cake by flattening the top, cutting the cake in half and stacking it one on top of the other with a layer of buttercream between. You can add food coloring to your frosting!

Leftover cake will keep well covered tightly in the refrigerator for 3 days.