Instructions

Preheat oven to 350 degrees F (175 degrees C). Line a muffin tin with paper liners.
Whisk flour, baking powder, and salt together in a bowl.
Combine brown sugar, peanut butter, and butter in another bowl; beat together with an electric mixer until creamy. Beat in egg and vanilla extract. Mix in flour mixture and milk; beat until smooth. Fold chocolate chips into the batter.
Spoon batter into the prepared muffin tin.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 20 to 25 minutes. Transfer to a wire rack to cool, about 30 minutes.