Combine chocolate chips and coconut oil in a microwave-safe mug; heat in the microwave until melted, about 30 seconds.
Whisk water, almond flour, coconut flour, and baking soda into the chocolate mixture until well combined. Add egg and brewed espresso; whisk until smooth.
Heat in the microwave until cake is cooked through, about 90 seconds. Let cool before serving, about 2 minutes.
