Preheat oven to 350º F.
Prepare three 9-inch round cake pans with nonstick baking spray or coat well with shortening or butter and flour the pans, taking care to remove all excess flour. Adjust time for other pan conversions (see conversion chart below recipe if you want to bake other sizes and have leftover batter in some cases, make cupcakes!)
In a medium-sized bowl. Sift together flour, baking powder, and salt, set aside.
Using a mixer, in another bowl, cream together butter and shortening until light and fluffy, approx 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 (s) 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 for 25-30 minutes, or until a toothpick or cake tester inserted in the center comes out clean. Note: Don't be tempted to keep opening the oven door a lot while the cake is baking as the oven temperature drops each time the door is opened.
Remove and allow to cool slightly in cake pans for about 5 minutes, then cool cake layers completely on a wire rack. Note: Leaving cake in cake pans too long could make the cakes stick to the pan and hard to remove because of the moisture that builds up in the pan while it cools.
Frost cake as desired.