1. Heat oven to 350°F. (325°F. for glass dish).
2. 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.
3. Bake 25 to 30 minutes or until lightly browned. Cool completely in pan on wire rack. Place 1 cup chocolate chips and shortening in small microwave-safe bowl. Microwave at MEDIUM (50%) 1 to 1-1/2 minutes or until smooth when stirred. Drizzle over top of bars. When drizzle is firm, cut into bars. Store loosely covered at room temperature. Makes about 24 bars.