1. Heat oven to 350°F. Grease 15-1/2x10-1/2x1-inch jelly-roll pan.
2. Stir together brown sugar, butter and granulated sugar in large bowl; beat in eggs and vanilla. Add flour and salt, beating just until blended. Set aside 2 tablespoons each milk chocolate chips and peanut butter chips. Stir remaining chips in batter; spread batter in prepared pan.
3. Bake 25 to 30 minutes or until wooden pick inserted in center comes out clean and surface is lightly browned. Cool completely; cut into bars.
4. Place remaining 2 tablespoons milk chocolate chips, remaining 2 tablespoons peanut butter chips and shortening in small microwave-safe bowl. Microwave at MEDIUM (50%) 30 seconds; stir. If necessary, microwave at MEDIUM an additional 15 seconds at a time, stirring after each heating, until chips are melted and mixture is smooth when stirred. Drizzle over bars. About 6 dozen bars.