In a bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add eggs, 1 at a time, beating well after each addition. Combine flour, cocoa, baking soda and salt; add to creamed mixture alternately with milk, beating until smooth after each addition. Pour batter into a greased and floured 13x9-in. baking pan. , Bake at 350° until a toothpick inserted in center comes out clean, 35-40 minutes. Cool on a wire rack. When cake is cool, dust with confectioners' sugar or top with your favorite frosting.