Mix together flour, brown sugar, baking powder, and salt in a large mixing bowl.
In a smaller bowl, whisk together milk, peanut butter, oil, egg, and vanilla. Add wet ingredients to dry and stir just until moistened. Fold in chocolate chips if desired.
Spoon batter into twelve well greased or paper lined muffin cups.
Bake at 350° for about 20 minutes, or until lightly browned. Let cool in pan for about 5 minutes, then remove to cooling racks.