1. Heat oven to 350°F. Line cookie sheets with parchment paper.
2. Stir together cookie mix and flour; add butter and egg yolk. Stir until crumbly. Add 3/4 cup toffee bits; work with hands until dough forms a ball. (Dough will feel very much like pie dough.)
3. Divide dough in half. One half at a time, flatten into square or rectangle approximately 1/4 inch thick on prepared cookie sheet. Cut rectangle into 36 pieces; do not separate dough.
4. Bake 10 to 12 minutes or until edges are light golden brown and center is set. Cool 3 to 5 minutes; recut pieces and separate slightly. Slide parchment paper with cookies to wire rack. Cool completely. Repeat with remaining dough.
5. Line tray with waxed paper. Place milk chocolate chips and shortening in microwave-safe cup. Microwave at MEDIUM (50%) 1 minute; stir. If necessary, microwave at MEDIUM an additional 15 seconds at a time, stirring after each heating, until chocolate is melted and smooth when stirred.
6. Dip about half of each cookie into melted chocolate. Shake gently and slide cookie bottom against edge of container to remove excess chocolate. (If necessary, reheat chocolate at MEDIUM for a few seconds.) Place on prepared tray. Before chocolate sets, sprinkle with remaining toffee bits. Store in cool, dry place. Makes 6 dozen cookies.
Tip: Any leftover chocolate and toffee bits can be used to coat pretzel rods or other cookies.