Preheat oven to 350 degrees F.
Line a 12 cup cupcake pan with cupcake liners.
Add all-purpose flour, cake flour, baking powder, and fine sea salt to a small bowl. Whisk to combine. Set aside.
Add unsalted butter to stand mixer. Using paddle attachment, beat on medium speed for 2 minutes.
Add granulated sugar to stand mixer. Cream butter and sugar for 3 minutes. Scrape down the bowl, if necessary.
Add whole egg to sugar mixture. Beat on medium until incorporated, about 30 seconds.
Add egg whites to sugar mixture. Beat on medium until incorporated, about 30 seconds.
Add vanilla bean paste. Beat just long enough to incorporate.
Add half the flour mixture and beat on low until incorporated. Add the milk and beat on low until incorporated. Add remaining flour mixture and beat on low until incorporated.
Divide the cupcake batter evenly between the cupcake liners.
Bake in preheated oven for 22 minutes, or until a toothpick inserted comes out clean.
Remove cupcakes from oven and cool several minutes before removing cupcakes from muffin tin.
Cool completely before frosting.