Prepare brownie mix according to instructions on box. Pour one-half of mixture into a greased 7 x 11 baking pan.
Mix all ingredients for peanut butter layer. Spoon mixture evenly over one-half of brownie mixture forming a layer. (If mixture is too thick to spread, add one tablespoon of milk.).
Pour remaining brownie mix batter over and spread evenly.
Bake in a 350 degree oven for 30 minutes.
For Optional frosting: Melt chocolate chips and butter in microwave just until almost melted. Remove and immediately spread on top of brownies. Place in refrigerator for quicker cooling before cutting into bars.