Preheat oven to 350 degrees. Add paper liners to a cupcake pan then set aside.
Add cake flour, baking powder, baking soda and salt to a mixing bowl. Whisk until combined then set aside.
Beat unsalted butter and granulated sugar with an electric mixer until creamy and smooth — approximately 3 minutes. Scrape down the sides of the bowl with a spatula as needed.
Beat in egg whites and vanilla extract until incorporated.
Add sour cream then beat until combined.
Reduce mixer speed to low then add dry ingredients to wet. Beat until just combined–no longer or cupcakes will be dry.
Pour milk into the bowl then continue beating on low until batter is lump free. Avoid overmixing.
Split batter evenly between two bowls. Add cocoa powder to one of the bowls and whisk until smooth.
Place a heaping tablespoon of chocolate batter into cupcake liners.
Top chocolate batter with a heaping tablespoon of vanilla batter. After the addition of vanilla batter, cupcake liners should be 2/3 of the way full.
Bake cupcakes for 20 minutes or until a toothpick inserted into the center comes out clean. Avoid overbaking.
Remove pan from oven and allow cupcakes to cool completely before frosting with strawberry buttercream.
Repeat baking process for remaining batter.