Place the butter in a 12 oz. microwave safe mug and microwave about 30 seconds, or until melted. Remove 1 tsp. of melted butter and set aside for the topping.
If the butter is hot, allow it to cool to room temperature.
Break the egg into the mug with the butter and beat well.
Stir in 1 Tbsp. espresso.
Add the Keto Chow, 3 teaspoons erythritol, coconut flour, and baking powder. Stir until combined.
Gently place the chocolate chips together on the surface of the batter. They will sink in as the cake cooks.
Place the mug in the microwave and cook on full power for 60-90 seconds or until the cake is doubled and shrinks away from the sides. Be careful when handling; it will be very hot.
Let the cake cool for two minutes before removing it from the microwave.
In a small bowl stir together the reserved butter and erythritol with 1/4 teaspoon espresso to make a glaze.
Spread the glaze over the cooled mug cake and garnish with whipped cream if desired.