Preheat your oven to 350°F. Grease three 6" round cake pans (or two 8" round cake pans), and/or line them with round parchment cake liners.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar together on medium-high speed until light and fluffy--about 3-4 minutes.
Add the egg whites, one at a time, followed by the vanilla and continue beating until creamy. Pause to scrape down the sides of the bowl as needed.
In a microwave safe bowl or glass, whisk the buttermilk and milk together. Microwave them for 20 seconds, or until no longer cold to the touch.
In a separate bowl, sift together the flour, baking powder, baking soda, and salt.
While running the mixer on low speed, add half of the flour mixer to the bowl, followed by all of the milk, and then the remaining flour. Pause the mixer and use a flexible spatula to finish folding the ingredients together until the flour is just combined.
Divide the batter evenly between the three cake pans. Use a measuring cup to help portion it out evenly.
Bake for 25-30 minutes, or until the cakes are set and a toothpick comes out clean.
Let the cakes cool completely in the pan. Then, remove them, gently wrap them with plastic wrap, and place them into the freezer to chill for at least an hour (or overnight).