Preheat oven to 350 degrees F. Spray a 9×13 baking pan with a cooking spray.
In a bowl, together the flour, baking powder, and salt. This is the flour mixture.
In your stand mixer, beat the butter (about 1-2 minutes) until its light and fluffy. Gradually add the sugar and beat until light and fluffy.
Add eggs, one at a time, beating well after each addition. Scrape down the sides of the bowl as needed.
Add the vanilla extract and beat until combined.
Then alternatively add the flour mixture and the milk, and end with flour mixture. So it should be flour, milk, flour, milk and flour.
Bake 30-35 minutes or until a toothpick inserted into the center of the cake comes out clean.
Place the cakes on a wire rack to cool for 10 minutes. Then invert the cake.
Vanilla Buttercream
Beat butter on a medium speed for 30 seconds until smooth.
Mix in vanilla extract and salt on a low speed.
Slowly add in 7 cups of powdered sugar, 1 cup at a time. Half way through add in 3 Tbsp of heavy cream or milk to make it easier to mix.
Mix on low until the ingredients are fully incorporated and the desired consistency is reached.
If the frosting is too thick, add in 1 tbsp of cream (1 Tbsp at a time). If the frosting is too thin, add 1/4 cup of powdered sugar. Add the desired color.