Prepare the Pans: Preheat oven to 350°F (177°C). Grease and line two 9-inch cake pans with parchment paper.

Mix Dry Ingredients: In a medium bowl, whisk together cake flour, baking powder, baking soda, and salt. Set aside.

Cream Butter and Sugar: Using a mixer, beat softened butter with sugar until light and fluffy, about 3 minutes.

Add Egg Whites: Beat in the egg whites until well incorporated. Scrape bowl as needed.

Blend Wet Ingredients: Add sour cream and vanilla, mixing until combined.

Combine Dry and Milk: Gradually add dry mixture while pouring in milk on low speed. Mix gently until smooth.

Bake the Layers: Divide batter evenly between pans. Bake for 24–25 minutes, or until a toothpick comes out clean.

Cool the Cakes: Place pans on a cooling rack for 1 hour. Remove cakes carefully and cool completely.

Make Buttercream: Beat butter until creamy. Add confectioners’ sugar, heavy cream, vanilla, and salt. Beat for 2 minutes until fluffy.

Assemble the Cake: Trim tops for a flat surface. Frost the first layer, stack, and add a crumb coat. Chill for 20 minutes, then frost fully.