Stir together sugar, flour, baking powder, and salt in a mixing bowl and set aside. (Or transfer to a mason jar for storage or gifting.)
Vanilla Cake
Preheat oven to 350 degrees F. Grease two 9-inch cake pans and set aside.
Cream together butter, egg whites, and vanilla on low speed until smooth.
Add in 1/2 of the dry cake mix ingredients and mix until just combined. Add the milk and mix again. Add remaining dry ingredients and mix until combined.
Add sour cream and beat until completely mixed together.
Divide batter evenly between the two cake pans.
Bake for 25 minutes or until a toothpick inserted in the center comes out clean.
Remove from oven and cool completely. Ice with vanilla frosting and cut into 12 slices.
Notes
This recipe will make two 9-inch cakes or one 9-inch x 13-inch cake.
Servings and nutrition will vary by how you cut the cake.
If you are making this cake from scratch, and not using the premade mix:
Mix the flour, baking powder, and salt together and set aside.