Combine first 5 ingredients in a medium bowl, stirring well. Stir in milk, margarine, and vanilla; stir with a wire whisk until blended. Pour batter into a 1-quart baking dish coated with cooking spray.
Combine 1 tablespoon sugar and 1 1/2 teaspoons cocoa; sprinkle evenly over batter. Pour hot water over sugar mixture. Bake at 350° for 18 minutes. Remove from oven, and let stand 10 minutes. Spoon cake evenly onto 2 dessert plates. Top each serving with 1/4 cup ice cream.