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.
Notes
For this recipe, you could garnish it with whatever you would like, but it would be delicious with some Reese's Peanut Butter Cups on top, or melted into the cake.