Preheat oven to 350ºF. In a large mixing bowl, mix in peanut butter and melted butter, stir until well blended. Mix in sugar, eggs, and vanilla. Combine flour. And to peanut butter mixture along with milk and chocolate chips. Stir just until combined. Spoon batter into 12 paper lined 2 1/2 to 3 inch muffin tins. Bake 20 to 25 minutes or until muffins spring back when lightly touched in center. Serve warm or room temperature. Makes 12 muffins.