Preheat the oven to 350F degrees. Line a regular sized muffin tin with muffin/cupcake papers.
In a medium-sized bowl, sift together the cake flour, baking soda & salt. Give them a whisk & set aside.
In a large bowl using an electric mixer on medium speed, beat together the butter and sugar until light and fluffy.
Beat in the egg whites and vanilla extract.
In a liquid measuring cup, whisk together the sour cream and buttermilk.
Beat about 1/2 of the sour cream mixture into the butter mixture.
Beat about 1/2 of the flour mixture into the batter.
Beat in the rest of the sour cream mixture, followed by the rest of the flour mixture. Only beat the batter until combined. Do not overmix.
Spoon the batter into the prepared muffin pan, filling each about 2/3 full.
Bake on the middle rack of the preheated oven for 16-20 minutes, or until an inserted toothpick comes out clean and the cupcakes feel slightly firm to the touch.