Preheat your oven to 350 degrees F. Spray two 9-inch round cake pans with cooking spray. Set aside.
Prepare the cake batter according to the package directions.
Place a medium saucepan over medium heat and add the brown sugar, butter, and heavy whipping cream. Stir constantly. Remove from the heat as soon as the caramel sauce is combined and melted. (The sauce will continue to cook in oven.)
Divide caramel sauce evenly between the pans. Tilt the pans to spread the sauce over the bottom of each pan. Sprinkle the chopped nuts over the caramel, 1/2 cup for each pan.
Gently add half of the cake batter to each pan in dollops so it doesn't spread the caramel and peanuts to the sides of the pans.
Bake the cakes per the box directions, about 23-29 minutes, until a toothpick inserted in the center comes out clean.
Remove from the oven and allow the cakes to cool in the pans for 5 minutes. Run a knife around the edge of the pans to loosen the cakes.
Cut off any bump that may have formed while baking to level the tops of the cakes. Place a plate on top of the cake pan and flip over. Allow the pan to sit upside down on the plate to cool.
While the cake is cooling, prep the Cool Whip. Regular whipped cream will collapse under the cake layers, so you need Cool Whip or stabilized whipped cream.
Once the cakes have cooled completely, lift the cake pans off the cakes. Add half the Cool Whip to the caramel side of one cake. Place the second cake layer on top of the first and top with the remaining Cool Whip.
Cut the Snickers bars into chunks. Spread over the top of the cake. Refrigerate until ready to serve.