In a large microwave-safe mug, whisk together flour, sugar, cocoa powder, baking powder, and a pinch of salt until evenly distributed.

Pour in milk, oil, and vanilla extract, stirring gently until the batter looks smooth and slightly glossy.

Place the mug on a microwave-safe plate. Cook on high power for 60 seconds, then check for doneness.

If the center still looks wet, microwave in 10-second bursts, up to a total of 1 minute 30 seconds.

Let the mug cake cool for one minute, then top with chocolate chips, whipped cream, or berries.