Make the batter
Spray a large heatproof mug with cooking spray. To a small bowl, add the milk, oil, and vanilla and whisk to combine. Add the flour, cocoa powder, sugar, baking powder, baking soda, and salt. Whisk to combine, but do not overmix.

Layer with peanut butter in the mug
Pour half of the batter into the prepared mug. Dollop the peanut butter on top, then add the remaining batter.

Microwave and serve
Microwave on the standard high setting for 1 minute and 10 seconds or until the batter is just barely set on top.

VERY IMPORTANT: Do not overcook, or the cake will turn out tough and dry! If the batter is still wet, microwave in 5-second intervals until it’s done.