grease bottom and sides of pan (I use 2 round 10inch pans or 1 9 by 13). preheat oven to 350 degrees. I would even suggest lining the bottom with parchment paper and then spraying.
Combine flour, cocoa, baking soda, baking powder, and salt. Set aside.
In a separate bowl beat butter for 30 seconds or so. slowly add sugar.
Add eggs 1 at a time beating after each one. Beat in vanilla.
Add dry ingredients beating after each addition.
Add ½ of the milk, beat. Then add the other ½. Beat just until everything is combined nicely.
Spread batter about into pans making them about ⅔ full. You may have remaining batter (I sometimes make a few cupcakes.)
Bake for 30-35 minutes for round pans, 5 min longer or so for the 9 by 13 pan. Watch it though! a knife coming out clean from the center means it is done. NOTE: Many readers are finding they need to increase the baking time. This is probably due to the difference in ovens, pans, and elevation. Always go by the way the cake looks and if the cake is done in the center.