Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9 inch round cake pan.
In a medium bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, cocoa, baking powder and salt; stir into the creamed mixture alternately with the milk.
Pour batter into prepared pan. Bake for 30 to 35 minutes in preheated oven, or until a toothpick inserted into center of cake comes out clean.