Preheat your oven to 350°F (175°C). Grease and flour four 6-inch round cake pans.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
2 1/2 cups all-purpose flour, 1 tablespoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt
Using your stand mixer with the paddle attachment or a large bowl and hand mixer, mix together the granulated sugar, oil, eggs, egg whites, and vanilla extract until light and fluffy.
1/2 cup sunflower oil, 1 3/4 cups granulated sugar, 2 large eggs, 2 egg whites, 1 tablespoon vanilla extract
Alternating with the milk, gradually add the dry ingredients to the wet ingredients, beginning and ending with the dry ingredients, mixing just until combined.
3/4 cup whole milk
Gently fold in the sour cream just until combined.
3/4 cup sour cream
Divide the batter evenly between the prepared pans. I recommend using a scale for this to make sure each layer is the same size. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely. Once cooled, remove the cakes from the pans.