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.

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.

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.

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.

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.