Preheat the oven to 350°F. Line a 12-cup muffin pan with cupcake liners. Line a second pan with 3 liners – this recipe makes about 15 cupcakes. Set aside.
Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.
Using a 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 and vanilla extract. Beat on medium-high speed until combined, then beat in the sour cream. Scrape down the sides and up the bottom of the bowl as needed.
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.
Frost cooled cupcakes with vanilla buttercream. I piped it on using a Wilton 2A tip. Because of the morsel speckles, I recommend using a larger tip.
Leftover cupcakes keep well covered tightly in the refrigerator for 3 days.