In a small bowl, whisk flour, cocoa powder, sugar and baking powder until completely combined. Whisk in milk, butter and vanilla until smooth. Fold in chocolate chips. Pour batter into a large microwave-safe mug.
Dollop peanut butter into center of mug, gently pressing down until even with top of batter. Microwave on high for 1 minute. Allow to cool for 1 minute before serving.
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