Preheat the oven to 350°F. Take a moment to grease and flour two 9-inch cake pans, then line the bottoms with parchment paper. This preparation ensures your cake will release easily after baking.
In a small bowl, whisk together the egg whites and 1/4 cup of buttermilk. Set this aside for a moment.
In a larger mixing bowl, beat the softened butter and granulated sugar together until they are creamy, which should take about 2 to 3 minutes. Add in the vanilla extract to enhance the flavor, then sprinkle in the dry ingredients: cake flour, baking powder, and salt. Mix these until they are well-combined. Slowly pour in the egg white mixture to incorporate everything smoothly.
Beat in the remaining buttermilk for an additional 1 to 2 minutes, making the batter light and fluffy.
Pour the batter evenly into your prepared cake pans. Bake in the preheated oven for about 25 to 28 minutes. To check if your cake is done, insert a toothpick into the center; it should come out clean. Once baked, allow the cakes to cool in their pans for 5 minutes before inverting them onto wire racks to cool completely.
For the frosting, beat the softened butter for 3 to 4 minutes until it becomes pale. Gradually add powdered sugar, vanilla extract, and milk, continuing to beat for another 3 to 4 minutes until the frosting is fluffy and smooth.
To assemble your lovely cake, place one cooled layer on a cake plate and generously spread frosting over the top. Carefully place the second layer on top and frost the sides and top of the cake with the remaining frosting.