Prepare the Mug. Use a microwave-safe mug that holds at least 12 ounces to prevent overflow. Lightly grease the mug with a bit of cooking spray or butter for easy cleanup.
Mix the Dry Ingredients. In the mug, whisk together flour, sugar, cocoa powder, baking powder, and salt until well combined.
Add the Wet Ingredients. Pour in the milk, brewed coffee, vegetable oil, and vanilla extract. Stir until the batter is smooth and lump-free. If using chocolate chips, fold them in now.
Microwave the Cake. Microwave on high for 60-90 seconds, depending on your microwave’s power. The cake is done when it has risen and the center is set but still slightly soft.
Let It Cool & Enjoy. Allow the cake to cool for one minute before adding toppings like whipped cream, powdered sugar, or chocolate drizzle.