In a microwave-safe mug (that holds at least 8 oz of liquid) add cocoa powder, coconut flour, baking powder and Swerve. Mix with a small whisk to break up any clumps and mix until evenly combined.
Add in heavy cream, water and oil. Whisk until batter is smooth. Sprinkle surface with dark chocolate.
Cook brownie in the microwave at full power for about 60-80 seconds. I like the brownie to be extra fudgy and slightly underdone, so I cooked mine to 65 seconds. Allow brownie to cool a few minutes before eating. Brownie is best eaten warm.