In a small bowl, combine hot water and 1/4 cup cocoa powder. Microwave on medium uncovered, for 20 seconds; set aside. In a mixing bowl, combine flour, sugar, baking soda, baking powder, salt, oil, egg, and vanilla extract. Add cocoa mixture. Mix until all is moistened. Pour batter evenly into a 7" round plastic cake pan. Microwave on high, uncovered, for three minutes and 30 seconds, or until toothpick inserted in center comes out clean, rotating once during cooking time. Meanwhile, to make the frosting, combine powdered sugar, margarine, milk, and remaining cocoa powder. Mix until spreading consistency. Spread over cooled cake.