In large bowl, combine cake mix with oil and eggs; blend well. Add in chocolate chips, chopped Heath bars, and additional toffee pieces (optional). Press 1/2 of this mixture into the pan. Bake for 10 minutes.
In medium saucepan, combine butter, caramels, and condensed milk. Stir constantly and cook over med-low heat until mixture is smooth. Pour caramel mix evenly over the pre-baked layer 1. Top with the remaining cake mixture, attempting to cover the entire carmel layer.
Bake for another 25-30 minutes or until the top is set and golden brown. Cool for 20 minutes then run a knife around the edges of the pan. Cool another 40 minutes. Refrigerate for 1 hour.