Preheat oven to 350º F.
Prepare two or three 9" round baking pans (or three 8" pans can also be used) or a 9×13 baking dish with nonstick baking spray or by coating with butter and lightly flouring, shaking away any excess flour and discarding. I like to add parchment paper to the bottoms of the pans and then spray with Baker's Joy. It's the best!
In a medium-sized bowl. Whisk together or sift together flour, baking powder, and salt, set aside.
Using a mixer, in another bowl, cream together butter and shortening until light and fluffy, approximately 3-5 minutes.
Gradually add sugar one cup at a time, making sure to fully incorporate each cup before adding another. Then start adding each egg one at a time, making sure to fully incorporate each egg before adding another, but gently incorporate in, don't over beat.
Pour milk of choice and vanilla into measuring cup and whisk together until mixed.
Add creamed mixture alternately with milk mixture and the dry mixture, beginning and ending with dry ingredients.
Gently stir all ingredients until combined, getting all ingredients from side of mixing bowl mixed in as well, but do not over-mix!
Evenly distribute cake batter between cake pans and place pans into oven.
Bake until the cake springs back to the touch and a toothpick or skewer inserted into the middle of the cake comes out clean, about 30-35 minutes for two 9-inch pans, 25-30 minutes for three 9-inch pans, 35-40 for three 8-inch pans and 35-40 minutes for a 9×13 baking pan. Remove from the oven and allow to cool about 8-10 minutes in the pan placed on a wire cooling rack. Then, turn the cakes out on the cooling rack to cool completely.
Frost cake as desired.