In bowl, mix 1 1/2 cups graham crackers, crushed, with 1 lb. box powdered sugar. Add 1 1/2 cups extra chunky peanut butter and add 2 sticks melted butter. Mix well. Pat into a 9 x 13 buttered pan. Melt 12 oz bag chocolate chips -spread on top. Put in fridge for 10-15 mins, then cut into small squares. Return to fridge until firm.