Preheat the oven to 325 degrees F, mist three 6-inch cake pans* with non-stick spray, and line them with circles cut from parchment paper. Place the cake flour, all-purpose flour, sugar, baking powder, baking soda, and salt in the bowl of an electric mixer, and stir on low speed to combine.
Add the butter, mixing on medium-low speed until the mixture resembles damp sand (about 1 minute).
Mix in the eggs and the additional yolk until fully incorporated.
Scrape the bottom and sides of the bowl with a silicone spatula, then add the sour cream, milk, and vanilla.
Mix on medium speed for 2 to 3 minutes, to aerate the batter and build the cake's structure.
Divide the batter equally between the prepared pans, and bake until the edges are golden brown, the cake feels springy when pressed, and a toothpick inserted in the thickest partcomes out clean (approximately 35 to 45 minutes).
Cool completely before filling and frosting.