Preheat oven to 350 degrees and line a cupcake pan with 12 liners. Then set aside.
In a small bowl sift together the cake flour, baking powder, baking soda salt. Then set aside.
1 ¼ cup (140 g) cake flour, spooned and leveled, ¾ teaspoon baking powder, ⅛ teaspoon baking soda, ¼ teaspoon salt
Add the softened butter and granulated sugar to a large bowl. Cream together with an electric mixer on high speed until fluffy, 1-2 minutes.
5 tablespoon (70 g) unsalted butter, softened, ¾ cup (150 g) granulated white sugar
Then add in the egg whites and vanilla. Mix on medium speed until smooth and pale in color, 1-2 minutes.
2 egg whites, at room temperature, 1 ½ teaspoon vanilla
Alternate adding the buttermilk and the dry ingredients to the butter mixture. Mix on low speed until everything is incorporated. Scrape the sides of the bowl as necessary.
½ cup (120 ml) buttermilk, at room temperature
Fill each liner with batter. Each should be about ¾ of the way full.
Bake the cupcakes for 16-18 minutes, or until a toothpick comes out clean from the centers.
Let the cupcakes cool in the pan for ten minutes, then transfer them to a cooling rack to cool completely.