Mix flour, sugar, 1 ounce cup cocoa powder, and salt together in a bowl. Add egg, milk, melted margarine, and vanilla extract. Stir well to combine and transfer to a large microwave-safe bowl with sides at least 2 inches high.
Combine brown sugar and 1 ounce cocoa powder in a bowl. Sprinkle on top of the batter in the microwave-safe bowl. Pour warm water gently over the cake; use a fork to gently prick a few holes in the cake to allow water to seep in.
Cook in the microwave on high power for 5 minutes. Serve with ice cream or custard.