Preheat the oven to 350°F. Line a 9x9 baking dish with parchment paper. Be sure to allow for enough parchment paper to overhang the sides of the pan. This allows easy removal of the Snickers cake to frost, cut, and serve. Set aside.
In a large mixing bowl, stir together the packet of Devil’s food cake mix, ½ cup evaporated milk, melted & cooled butter, and the egg just until all ingredients are combined. The batter will be very thick.
Gently press half of the cake batter into the bottom of the parchment-lined pan. You may need to lightly wet your fingertips with cool water to help press the mixture into the pan. Bake for 15 minutes.
While the cake is baking, you will need to place your unwrapped soft caramel candies and evaporated milk into a medium saucepan, on medium-low heat, for 6 to 8 minutes or just until all the caramel candies are melted, and the sauce is smooth. Turn off the heat to the saucepan and allow the caramel sauce to cool slightly while the cake finishes its first bake.
Top the warm cake with semi-sweet chocolate chips, ½ cup chopped peanuts, and caramel sauce.
Drop large spoonfuls of the remaining batter over the top of the caramel. Using a spoon, or damp fingertips, just lightly try to spread the batter. This batter will spread as it bakes, so you do not have to be precise when spreading the batter over the top. You really just want to make sure that most of the surface area has batter on it before the final bake.
Return to the oven and bake for an additional 15 to 20 minutes or just until the top layer of cake is baked.
Remove the cake from the oven and allow it to cool completely on the counter before removing the cake from the pan, using the overhanging parchment paper to help pull the cake out of the pan. Frost the cake with the tub of milk chocolate frosting, and garnish it with a drizzle of bottled caramel sauce and additional ¼ cup chopped peanuts before slicing and serving.