Preheat oven to 350 degrees F. Grease a 9×13 baking dish with cooking spray.
In a stand mixer, mix together sugar, flour, cocoa powder, baking soda, baking powder and salt.
Add the eggs, vanilla, milk, and oil and combine.
Add the boiling water and put the mixer on low until everything is combined.
Scrape the bottom of the bowl with a spatula and mix everything together. The batter will look thin and this is normal.
Pour the batter into the greased baking dish. Bake for about 30-35 minutes. Cool on a wire rack. The cake needs to be complete cooled before mixing with the frosting.
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.