Lightly butter two small microwave-safe mugs.
Whisk together the 1/2 cup milk, 1/2 cup sugar, 1/4 cup oil, 1 large egg and 1/4 tsp vanilla in a large bowl until smooth, about 2 minutes.
Whisk in 1/2 cup flour, 2 tablespoons cocoa powder, 1/4 tsp baking powder and 1/8 tsp salt until just combined.
Pour 1/2 of batter into each mug. Microwave, one at a time, until puffed and a toothpick inserted into the center comes out clean, about 2 minutes 50 seconds.
Note: Continue microwaving in 15-second intervals, if needed. Let cool slightly. Top as desired.