Grease a round microwave-safe dish; line bottom with plastic wrap. In a small bowl, combine cocoa and 1/3 cup water. Microwave on HIGH for 40-50 seconds or until slightly thickened.
In a medium bowl, combine flour, sugar, baking soda and powder, and salt. Add oil, remaining 1/3 cup hot water, egg, vanilla and chocolate mixture. Beat until batter is smooth and well blended. Pour batter into prepared pan. Microwave on HIGH 5-6 minutes* without turning until cake beings to pull away from sides. (Some moist spots may remain but will disappear on standing.) Let stand for 5 minutes; invert into cake tray. Peel off plastic wrap; let cool. Frost if desired.
*Time is for 600-700 watt microwave oven