Preheat your oven to 400 degrees Fahrenheit and prep your cupcake pan with liners + spray.
In a bowl, whisk together the all-purpose flour, cake flour, baking power, baking soda, and sea salt.
In the bowl of an electric mixer (using a paddle handle attachment), beat together the butter and sugar on medium-high speed for about 1-2 minutes, until light and fluffy. Scrape down the sides as needed using a rubber spatula.
Add the egg whites and vanilla, reducing the speed a little at this point. Add in sour cream until well combined.
Now, reduce the speed to low and slowly add in the dry ingredients. Next, add in the milk and mix until incorporated. NO OVER-MIXING!
Scoop the batter into each cupcake liner (3/4 full) and bake at 400 degrees Fahrenheit for the FIRST 5 minutes. Then reduce temperature to 350 degrees Fahrenheit and continue baking for another 14-17 minutes or until a toothpick comes out clean in the center.
Once done, turn off oven and keep cupcakes inside with the oven door slightly open for about 2 minutes before taking them out.
Let them cool COMPLETELY before frosting.