Preheat oven to 400 degrees F (200 degrees C). Line muffin cups with paper liners.
Whisk flour, sugar, oats, cocoa powder, and baking powder together in a bowl.
Combine milk and peanut butter in a saucepan; cook and stir over medium-low heat until mixture is smooth, about 5 minutes. Remove saucepan from heat and slightly cool peanut butter mixture, about 5 minutes. Stir peanut butter mixture into flour mixture until batter is just combined; fold in chocolate chips. Spoon batter into prepared muffin cups, about 2/3 full.
Bake in the preheated oven until a toothpick inserted in the center of a muffin comes out clean, about 13 minutes.