Add the flour, sugar, cocoa powder, and baking powder to a large microwave-safe mug. Stir until well combined.
Pour in the milk, vegetable oil, and vanilla extract (if using). Mix until a smooth batter forms, making sure there are no dry pockets of flour.
Stir in the chocolate chips or place them in the centre of the batter for a gooey chocolate middle.
Microwave on high for 60–90 seconds, depending on the power of your microwave. The cake should have risen and look just set on top.
Let the mug cake rest for 1 minute before serving, as it will continue to cook slightly and cool to a safe temperature.
Enjoy straight from the mug, or top with ice cream, whipped cream, chocolate sauce, or fresh berries for an extra-special treat.