Step 3: Prepare the oven. Preheat it to 350 degrees F or 180 degrees C. Using parchment paper or silicone liners, line three 6-inch cake pans, and grease them with baking spray.
Step 4: In a large mixing bowl or bowl of a stand mixer, place the flour, sugar, baking powder, baking soda, and salt. Mix well. Then, add the butter and mix until you have a mixture that resembles coarse crumbs. Stir in the oil.
Step 5: Next, add the buttermilk and beat until smooth before adding the vanilla.
Step 6: Slowly add the egg whites while the mixer is running on low, scraping the sides of the bowl as needed. Mix until the batter is smooth.
Step 7: Into the prepared cake pans, equally divide the batter. Place in the preheated oven and bake for about 12 to 15 minutes or until a toothpick inserted in the middle of the cakes comes out clean.
Step 8: Remove from the oven when done and allow the cake layers to cool for about 10 minutes in the pan before inverting to a wire rack to cool fully.