Step 1

Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners. Line a second pan with 2 liners—this recipe makes about 14 cupcakes. Set aside.

Step 2

Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

Step 3

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed until smooth and creamy, about 1 minute. Add the sugar and beat on high speed for 2 minutes until creamed together. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add the egg whites, vanilla extract, and vanilla bean and beat on medium-high speed until smooth and combined, at least 2 minutes. Scrape down the sides and up the bottom of the bowl as needed and then beat in the sour cream. With the mixer on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, slowly pour in the milk until combined. Do not over-mix. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. The batter will be slightly thick.

Step 4

Pour/spoon the batter into the liners—fill only 2/3 full to avoid spilling over the sides. Bake for 19–22 minutes, or until a toothpick inserted in the center comes out clean. For around 30–36 mini cupcakes, bake for about 11–13 minutes, same oven temperature. Allow the cupcakes to cool completely before frosting.

Step 5

Frost cooled cupcakes with vanilla buttercream. You can swipe the frosting on with an icing knife or use a piping tip such as Wilton 1M. Leftover cupcakes keep well covered tightly in the refrigerator for 3 days.