Whisk and sift dry ingredients: Sift all-purpose flour and cake flour into a medium mixing bowl. Add baking powder and salt and whisk 20 seconds, set aside.
Cream together mixture: In the bowl of an electric stand mixer fitted with the paddle attachment that constantly scrapes bowl, cream together butter, granulated sugar and seeds of 1 vanilla bean until mixture is pale and fluffy. Mix in egg, then mix in egg whites one at a time. Blend in vanilla extract.
Mix in dry ingredients alternating with milk: With mixer set on low speed add flour in three additions alternating with 1/2 of the milk in two addditions.
Fold batter and divide among cups: Remove bowl from stand mixer and scrape bowl and gently fold batter with a spatula to ensure everything is evenly combined. Divide batter among paper lined muffin cups filling each cup 2/3 full.
Bake until set: Bake in preheated oven until a toothpick inserted into center comes out clean, about 17 - 20 minutes.
Cool completely: Remove from oven, transfer cupcakes to a wire rack to cool (after about 5 - 10 minutes of cooling I like to transfer my cupcakes to an airtight container to finish cooling so they don't dry). Once cool frost with Vanilla Bean Buttercream Frosting. Store cupcakes in an airtight container.
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