Preheat oven to 275° F. Grease large roasting pan or two 13x9x2-inch baking pans with butter.
Place popcorn and peanuts in prepared pan or pans.
Combine toffee bits, corn syrup, and salt in heavy medium saucepan. Cook over medium heat, stirring constantly, until toffee is melted (about 12 minutes). Add the vanilla, then pour over popcorn mixture; stir until evenly coated.
Bake 30 minutes, stirring every 10 minutes. Remove from oven; stir every 2 minutes until slightly cooled (this helps break it up). Cool completely and spread popcorn on a parchment lined pan.
Place milk chocolate chips and shortening in small microwave-safe container or in a heavy duty freezer bag or disposable decorating bag. If using a zipper bag, make sure it's the heavy duty type.
Microwave at 50% power for 1 minute; stir, or if using a bag, massage the bag. Microwave at at 50% power an additional 10 seconds at a time, stirring (or massaging bag) after each heating, until chocolate is melted and smooth. Drizzle melted chocolate over popcorn. If using a bag, snip the corner off the bag and squeeze lines of chocolate over the popcorn. Allow drizzle to set up; break into pieces.
Store in tightly covered container in cool, dry place.