Preheat the oven to 350 degrees Fahrenheit. Afterward, grease and line the cake pans.
Then, grab a mixing bowl and whisk together the all-purpose flour, cake flour, baking powder, baking soda and salt.
After that, in a mixing bowl of your hand or stand mixer fitted with a whisk attachment, cream the butter and beat on medium speed until smooth. Afterward, at a low rate, slowly add the sugar and beat to medium-high speed for two minutes until fluffy. Then, add the vanilla extract, and use a rubber spatula to scrape the excess butter at the bowl’s corners and surface.
At low speed, gradually add the egg whites, turn the mixer on high speed and beat the cake batter for two minutes, or until it is well mixed. After that, beat in the vanilla extract and sour cream. Also, scrape down the sides of the bowl as often as needed.
Then, in a sequence, alternately add in the flour mixture and milk at medium speed. After that, ensure to scrape down the bowl’s sides properly and whisk until the cake batter is thoroughly mixed.
Scrape the corners and fold the batter thoroughly before dividing the cake batter into the cake pans.
Bake at 350 degrees Fahrenheit for twenty-five to thirty minutes, or until a toothpick inserted in the middle has a few moist crumbs stick to the toothpick. Allow the cake to cool completely in the pans and set on a wire rack before cutting or frosting.