For the cupcakes: Preheat oven to 350 degrees. Line a 12-cup muffin pan with paper liners.
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.
For the frosting: In the bowl of an electric stand mixer cream together butter and seeds of one vanilla bean until butter is fluffy. Add in powdered sugar, cream and vanilla and whip until frosting is light and fluffy***.
Preferably use a kitchen scale to measure flour. If you don't own a kitchen scale, to measure flour scoop from package or container using a measuring cup and level top with a butter knife.
Just measure 2/3 cup milk and remove 1 Tbsp.
Thin with a little cream if needed or thicken with a little powered sugar if needed.