Preheat the oven to 350F. Line a muffin tin pan with cupcake liners and set it aside.
In a large bowl, whisk together the cake flour, baking powder, baking soda and salt. Set it aside.
Using either a hand mixer or a stand mixer fitted with a paddle attachment, beat the butter on high speed until smooth and creamy, about 1 minute. Then beat in the butter for about 2 minutes, until they are creamed together. Beat in the egg whites, vanilla extract and vanilla bean seeds. When that's all combined, beat in the sour cream. Scrape down the sides of the bowl as needed.
Switch to low speed and slowly add in the dry ingredients. Then slowly add in the milk. Mix until just combined.
Fill each cup up 2/3 of the way full. Then bake for 17-22 minutes, or until a toothpick inserted into the center comes out clean. Remove the pan from the oven and let the cupcakes sit on the pan for another few minutes.
After about 5 minutes, move the cupcakes to a wire rack. Allow the cupcakes to cool completely before frosting them.
Mine are topped with vanilla buttercream, the recipe of which I have under my profile -->
Rose designs made using the Wilton 1M tip -->