Spray a medium microwave-safe bowl with nonstick cooking spray; set aside.

Combine sugar, milk, egg, butter, and vanilla in a mixing bowl; mix well. Combine flour, cocoa powder, baking powder, and salt in a separate bowl. Blend flour mixture into milk mixture until smooth. Pour into the prepared microwave-safe bowl.

Cover and cook in the microwave on high until cake springs back when touched, 2 to 2 1/2 minutes. Allow cake to cool for 5 minutes, then cover the bowl with a plate and invert cake onto the plate.