Whisk milk, vegetable oil, and sugar together in a 12 oz. or large mug.
Whisk in the flour until smooth.
Whisk in the oatmeal, baking powder, and salt until full incorporated.
Spoon 2 tablespoons of peanut butter on top and push slightly into the batter.
Microwave on HIGH for 30 seconds, and then in 15-second bursts until the top looks dry and cooked and springs back when pressed with a finger. Mine took about 90 seconds to cook.
Top with the remaining tablespoon of peanut butter and let stand at least 5 minutes before eating.
Notes
Slightly Adapted from The Kitchn. Throw a small handful of mini chocolate chips into the batter for extra fun!