In a large microwave-safe mug, combine the flour, sugar, cocoa powder, baking powder, and salt. Stir until well mixed.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Mix well until you have a smooth batter with no lumps.
If you're using chocolate chips, fold them into the batter now, ensuring they are evenly distributed.
Microwave the mug on high for about 1 minute and 30 seconds. Depending on the wattage of your microwave, you may need to adjust the time. The cake is done when it has risen and is set in the center but still moist.
Allow the mug cake to cool for a minute before serving. You can top it with whipped cream, ice cream, or a drizzle of chocolate sauce if desired.
Notes
Adjust microwave time based on wattage. Top with whipped cream or ice cream for extra indulgence.