Heat oven to 350°F. (325°F. for glass dish).
Melt butter in oven in 13x9x2-inch baking pan.
Sprinkle graham cracker crumbs over butter; pour sweetened condensed milk evenly over crumbs.
Top with 2 cups chocolate chips and peanut butter chips; press down firmly.
Bake 25 to 30 minutes or until lightly browned.
Cool completely in pan on wire rack.
In small microwave-safe bowl, place 1 cup chocolate chips and shortening.
Microwave at HIGH (100%) 1 to 1½ minutes or until smooth when stirred.
Drizzle over top of bars.