Preheat the oven to 350°F. Spray two 8 or 9- inch round cake pans or one 9x13- inch pan with non-stick spray.
In a medium bowl, stir together the flour, cocoa, baking powder and salt. In a larger bowl, beat the butter with an electric mixer for about half a minute, until it’s pale and creamy. Pour in the sugar and continue to beat for 3-4 minutes, until the mixture is light and fluffy. Add the eggs one at a time, beating well after you add each one. Scrape down the sides of the bowl whenever it needs it.
Stir the vanilla into the milk. Add about a third of the flour mixture to the butter mixture and stir it in by hand or with the electric mixer on low speed, just until it’s combined. Add about half the milk in the same manner, then another third of the flour, the rest of the milk and the rest of the flour, mixing just until the batter is blended.
Divide the batter between the greased cake pans and tap the bottoms a few times on the countertop to remove any air bubbles. To prevent a domed top, spread the top of the batter with a spatula, creating a slight dent in the middle and a raised edge. This compensates for the way a cake tends to rise higher in the middle.
Bake for 30-35 minutes, until the cakes are golden, the edges are pulling away from the sides of the pan and the tops are springy to the touch. Let them cool for about 10 minutes before running a knife around the edge of the pans and inverting them onto a wire rack. Cool completely before you frost them.