Preheat the oven to 350°F. Grease three 6x2 inch cake pans and line with parchment paper rounds. To make parchment paper rounds, trace the bottom of one of the cake pans on a piece of parchment paper and then cut out the circle. Place at the bottom of the cake pan. Grease the top of the parchment paper using cooking spray and set aside.
Whisk the flour, baking powder, baking soda and salt together in a medium bowl. Set aside.
Using a handheld mixer or a stand mixer fitted with the paddle attachment, cream the butter for about 1 minute on high until light and fluffy.
Add the sugar and beat on high for 2 minutes until the butter and sugar are well incorporated and creamy. Scrape down the sides of the bowl with a spatula as needed.
Add the egg whites and vanilla extract. Beat on medium high until well combined and then beat in the sour cream on medium high.
Slow the speed of the mixer to low speed. With the mixer still running, slowly pour in the dry ingredients and mix until just combined. Continue to run the mixer on low and slowly pour in the milk. Mix until just combined. If you need to, turn the mixer off and finish mixing the ingredients by hand to ensure they are well combined. Do not over mix.
Pour the batter evenly into the cake pans. Bake for 18-20 minutes, or until the cakes are baked through. The cakes are done when a toothpick inserted into the center of the cake comes out clean.
Remove cakes from oven and allow to cool on a wire rack in the pans completely before frosting.
If you live at high altitude (above 3,000 feet), please follow the instructions in the "Notes" section below!