Preheat your oven to 350°F (175°C).
Line a 12-cup muffin pan with parchment liners.
In a medium bowl, whisk together the cake flour, baking powder, and kosher salt. Set aside.
In a large mixing bowl, beat the unsalted butter and sugar together using an electric mixer on medium speed until light and fluffy, about 2-3 minutes.
Gradually add the egg whites to the butter mixture, mixing until fully incorporated after each addition.
Add the vanilla bean paste and pure vanilla extract, and mix until combined.
With the mixer on low speed, alternate adding the dry ingredient mixture and the room temperature milk, starting and ending with the dry ingredients. Mix until just combined.
Fill each liner about two-thirds full with batter.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the cupcakes to cool in the pan for about 5 minutes before transferring them to a cooling rack.
