Use a fork to stir together all ingredients except the chocolate chips in a 10 to 12 ounce microwave-safe mug.
Microwave on high until a toothpick inserted in the center comes out clean, about 1 minute 45 seconds. Pause cooking once halfway through and open the microwave door to prevent the cake from overflowing.
If desired, sprinkle the chocolate chips on top. Serve the mug cake while it's still warm.
Don't over-mix the batter! Like any cake, over-mixing will make it tough and rubbery instead of moist and fluffy.
Amp up the flavor. If you have instant espresso powder on hand, add 1/4 teaspoon of it to this recipe. It will help pull out the flavor notes in the chocolate.
Serve it how you like it. This cake is moist and delicious on its own, but you can serve it any way you like your chocolate cake! A few topping ideas are: chocolate chips, vanilla ice cream, whipped cream, or fresh sliced strawberries.