Step 1

Prepare Cake Layers: Preheat oven to 350°F. Grease (with shortening) 2 9-inch round can pans. Line the bottoms of each pan with parchment paper; grease paper with cooking spray. Dust pans with flour, tapping out excess.

Step 2

Prepare the cake mix according to package directions, replacing water with whole milk and adding the vanilla. Fold in the chopped candy bars. Divide batter between prepared pans.

Step 3

Bake in preheated oven until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Cool cakes in pans on a wire rack 10 minutes. Remove cakes from pans and transfer to a wire rack; let cool completely, about 30 minutes.

Step 4

Prepare the Frosting: Combine dark brown sugar, butter, and salt in a medium saucepan. Bring to a boil over medium-high heat, stirring frequently. Cook 2 minutes, stirring constantly, until the sugar dissolves and the mixture is smooth. Stir in half-and-half; return to a boil. Remove from heat and stir in vanilla. Cool 15 minutes. Gradually stir in enough powdered sugar to make a thick, spreadable frosting.

Step 5

Assemble cake: Place 1 cake layer on a serving plate; spread with frosting and sprinkle with chopped caramel candies and peanuts. Top with second cake layer; spread remaining frosting on top and sides of cake. Garnish with candy bars as desired.