In a large microwave-safe mug, mix together flour, brown sugar, cocoa powder, baking powder, and salt.
Add milk, vegetable oil, and vanilla extract to the mug. Stir or whisk until the mixture is smooth and there are no lumps. If using chocolate chips, mix them in.
Microwave on high for 60-90 seconds. Check if the cake is set but still moist on top. If not, microwave for an additional 15 seconds.
Serve warm with a scoop of ice cream or some whipped cream on top.