Heat oven to 350°F. Make and bake cake as directed on package for 13"x9" pan. Cool 15 minutes.
Poke top of warm cake every ½ inch with handle of wooden spoon. Drizzle milk evenly over top of cake; let stand until milk has been absorbed into cake.
Drizzle with caramel topping. Run knife around sides of pan to loosen cake. Cover and refrigerate about 2 hours or until chilled.
Spread whipped topping over top of cake. Sprinkle with toffee chips. Store covered in refrigerator.