Heat oven to 350 degrees. Line a 13x9 baking dish with parchment paper.

In the bowl of your stand mixer (or a large bowl if you're mixing by hand), beat together the sugar mix, butter, and egg. Stir in the Reese's cups. Press dough into the bottom of the pan. Bake for 18 to 20 minutes or until golden brown. Remove to a cooling rack.

Meanwhile, in a 2 quart saucepan heat the condensed milk and caramels until the caramels have melted, stirring constantly. Stir in the peanut butter and bring mixture to a boil. Boil for 2 minutes, stirring constantly. Remove from heat and stir in the peanuts. Spread the warm filling on the top of the warm cookie base. Cool completely, about 2 hours.

Spread frosting evenly over the caramel layer. Sprinkle with chopped peanuts if desired. Refrigerate for about 2 hours or until chilled. Cut into small bars, 9 rows by 4 rows. Store covered in the refrigerator. Allow to set at room temperature for 15 minutes before serving if you like the caramel layer to be soft.