Preheat oven to 350°F (177°C). Grease three 6×2 inch cake pans, line with parchment paper rounds, then grease the parchment paper.

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 on high speed until smooth and creamy, about 1 minute. Add the sugar and beat on high speed for 2 minutes until creamed together.

Add the egg whites and vanilla extract. Beat on medium-high speed until combined, then beat in the sour cream.

With the mixer on low speed, add the dry ingredients until just incorporated. Slowly pour in the milk until combined. Do not over-mix.

Pour batter evenly into cake pans. Bake for around 18-21 minutes or until the cakes are baked through.

Allow cakes to cool completely in the pans set on a wire rack.

Make the frosting: Beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, heavy cream, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes.

Assemble and decorate: Slice a thin layer off the tops of the cakes to create a flat surface. Place 1 cake layer on your cake stand, cover the top with about ¾ cup of frosting. Top with 2nd cake layer and cover with frosting. Top with the third cake layer and spread the remaining frosting all over the top and sides.

Refrigerate for at least 30-45 minutes before slicing.