1. Heat oven to 275° F. Grease large roasting pan (OR two 13x9x2-inch baking pans). Place popcorn in prepared pans.
2. Combine toffee bits and corn syrup in heavy medium saucepan. Cook over medium heat, stirring constantly, until toffee is melted (about 12 minutes). Pour over popcorn mixture; stir until evenly coated.
3. Bake 30 minutes, stirring frequently. Remove from oven; stir every 2 minutes until slightly cooled. Cool completely.
4. Spread popcorn on wax paper lined cookie sheet or pan. Place milk chocolate chips and shortening in small microwave-safe container. Microwave at MEDIUM (50%) 1 minute; stir. If necessary, microwave at MEDIUM an additional 10 seconds at a time, stirring after each heating, until chocolate is melted and smooth when stirred. Drizzle melted chocolate over popcorn. Allow drizzle to set up; break into pieces. Store in tightly covered container in cool, dry place. About 8 cups popcorn.