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.

In a medium bowl, whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar on high speed until light and creamy, about 3 minutes. Scrape down the sides of the bowl as needed. Add the egg whites, vanilla extract, and vanilla bean seeds (if using) and beat on high speed until smooth and well combined, about 2 minutes. Add the sour cream and beat on medium-high speed until combined, about 1 minute. With the mixer running on low speed, add the dry ingredients, and then slowly pour in the milk and beat just until combined. Do not over-mix. You may need to whisk it by hand a few times to make sure there are no lumps at the bottom of the bowl. The batter will be slightly thick. 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. Cool the cupcakes in the pan set on a cooling rack for 20 minutes, then remove them from the pan and return to the rack to cool completely before frosting.

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. Store covered in the refrigerator for up to 3 days.