In the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a handheld mixer, beat the butter until it’s pale in color. Add the sugar, vanilla flavoring and egg whites, and beat on medium-high speed for about a minute or until light and fluffy, stopping at least once to scrape the entire mixture off the sides and bottom of the bowl. To the large bowl with the butter and sugar mixture, add the dry ingredients in 3 equal portions, alternating with the buttermilk in 2 parts, beginning and ending with the dry ingredients and mixing to combine in between additions. Once all of the ingredients have been added, beat with the mixer until mostly combined, then turn over the batter a few times by hand with a spatula. It should be very thickly pourable and relatively smooth.