Peanut Butter Marbled Brownies
["Peanut Butter Filling (recipe follows)", "1 c. (2 sticks) butter or margarine", "2 c. sugar", "2 tsp. vanilla extract", "3 eggs", "3/4 c. Hershey's cocoa", "1 1/4 c. all-purpose flour", "1/2 tsp. baking powder", "1/4 tsp. salt", "1 c. Hershey's milk chocolate chips"]
["Heat oven to 350\u00b0.", "Grease a 13 x 9 x 2-inch baking pan. Prepare Peanut Butter Filling."]
["Peanut Butter", "butter", "sugar", "vanilla extract", "eggs", "cocoa", "flour", "baking powder", "salt", "chocolate chips"]