Spray a medium microwave-safe bowl with non-stick spray and set aside. In a mixing bowl combine the butter, sugar, egg, vanilla essence and milk; blend together. Combine the flour, cocoa powder, baking powder and salt; blend into the mixture until smooth. Pour into greased bowl.
Cover and cook in the microwave on high for 2 to 2 1/2 minutes or until cake springs back when touched. To serve, let the cake cool five minutes then cover the bowl with a plate and turn both bowl and plate upside down so the cake falls onto the plate.