Add all ingredients except the chocolate chips (if using) to an 8-ounce or larger microwave-safe mug. Use a fork to stir together until combined. It might seem lumpy at first but keep stirring and it'll resemble cake batter in no time!
Microwave on high until the cake has risen and springs back slightly when you poke it, about 60 seconds. If it's not done after 60 seconds, try cooking in 15-second intervals until done. (To bake in the oven, heat oven to 375 degrees Fahrenheit and place batter in an oven-safe ramekin or oven-safe mug. Bake until domed and the top springs back when you poke it, 15-20 minutes. )
Top with chocolate chips or chunks if desired and enjoy right away.