Preheat to 350°F. Prepare 12 cupcake tins with liners and set aside. Sift together cake flour, baking powder, and salt.
In the bowl of a stand mixer using the paddle mixing attachment, cream together room temperature butter and sugar for 2-3 minutes until combined, light and fluffy. Add the egg yolks and egg, along with the vanilla extract, and beat until combined. Slowly mix in sour cream until combined.
Add the dry ingredients and slowly stir in, adding milk once almost all dry ingredients are combined, and continuing to mix just until it's fully incorporated. Be sure to not overmix.
Fill the cupcake liners approximately 3/4 of the way full (I use 2 full scoops of this cookie scoop). Bake for 16-20 minutes until slightly golden brown and the cupcake bounces back when gently touched - or until the internal temperature reaches approximately 195-198°F. Let cool completely on a cooling rack before frosting.
To remove the center of the cooled cupcakes, use an apple corer and push it straight down the middle of the cupcake, approximately halfway down. Pull up and approximately 1 1/2 inches of the cupcake will be removed. Do this for the remaining cupcakes and eat or save the removed cake (no cake wasted!). See frosting instructions for filling.
Beat room temperature butter. Add powdered sugar, 1 cup at a time and beating inbetween additions. Incorporate 2 TBSP milk with the powdered sugar, as well as the vanilla extract. Add cocoa powder and remaining TBSP of milk. Beat until fully combined, light, and fluffy.
Place frosting in a large piping bag with a large round tip (or just have the pastry bag be your "tip"). Squeeze frosting into the hole in the cupcakes for the filling and continue squeezing the frosting around the top of the cupcake until covered. Use a knife or icing spatula to "swoop" the frosting. Frost the remaining cupcakes.
Serve immediately or store in a covered container at room temperature for 1-2 days.
To make cake flour: measure 1 1/2 cups all-purpose flour properly. Remove 3 TBSP of the flour and place it back in your canister. Add 3 TBSP corn starch and sift the flour and cornstarch together 2-3 times until fully combined.