1. Preheat the oven to 350 degrees. Line a cupcake tin with cupcake liners and set aside.
2. In a medium bowl, whisk the cake flour, baking powder, baking soda and salt together. Set aside.
3. Place your butter in a mixer and beat on high for about 1 minute until it’s creamy. Add the sugar and beat again on medium speed for a couple minutes until all combined. Scrape down the sides of the bowl as needed. Add the egg whites, vanilla extract and vanilla bean seeds and mix on medium until combined. Add the sour cream in next and mix again until combined.
4. Scrape down the sides as needed. Now add the dry ingredients a little at a time, mixing on low until just combined. Be sure not to over mix. Then put the mixer on low again and slowly add the milk. I mix for a few rotations and then finish it by hand.