Choose a microwave-safe mug that holds at least 12 ounces. Add the flour, sugar, cocoa powder, baking powder, and salt, then stir thoroughly with a fork until the dry mixture looks evenly colored.

Pour in the milk, oil or melted butter, and vanilla extract. Stir from the bottom and around the sides until no dry pockets remain, then fold in the chocolate chips if using.

Microwave the mug on high for 70 to 90 seconds, depending on the strength of your microwave. Stop when the cake has risen and the top looks mostly set, since it will continue cooking briefly from the residual heat.

Let the mug cake rest for one minute before eating. Add powdered sugar, ice cream, whipped cream, berries, or a drizzle of chocolate sauce, and serve while warm.