Heat the oven to 350°F (175°C). Grease two 8-inch round cake pans, line the bottoms with parchment paper, and lightly flour the sides if desired.

Whisk the cake flour, granulated sugar, baking powder, and salt together in a large mixing bowl. Add the softened butter and vegetable oil, then beat on low to medium speed until the mixture resembles damp sand or fine crumbs.

In a separate bowl, whisk together the room-temperature egg whites, buttermilk, and vanilla extract. Add this liquid mixture to the dry ingredients in two or three additions, mixing only until smooth after each addition and scraping down the bowl as needed.

Divide the batter evenly between the prepared pans. Bake for 25–32 minutes, until the tops spring back lightly and a toothpick inserted in the centers comes out with a few moist crumbs.

Cool the cakes in their pans for 10 minutes. Turn them out onto a wire rack, remove the parchment, and cool completely for at least 45–60 minutes before filling and frosting.