Melt candy bars and butter in a heavy saucepan over low heat about 5 minutes, stirring until smooth. Set aside.
Beat sugar and shortening at medium speed with an electric mixer about 3 minutes or until well blended. Add eggs, 1 at a time, beating just until blended after each addition.
Combine flour and salt. Stir together buttermilk and baking soda. Gradually add flour mixture to sugar mixture, alternately with buttermilk mixture, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in melted candy bar mixture and vanilla. Place 36 paper baking cups in muffin pans; spoon batter evenly into paper baking cups, filling two-thirds full.
Bake at 350° for 18 minutes or until a wooden pick inserted in center comes out clean. Remove cupcakes from pan, and let cool completely on wire racks. Spread cupcakes evenly with Chocolate-Marshmallow Frosting. Garnish, if desired, with chopped frozen fun-size chocolate-coated caramel and creamy nougat bars, candy corn.
Chocolate Marshmallow Frosting: Melt first 4 ingredients in a 2-quart saucepan over medium-low heat, stirring 5 minutes or until mixture is smooth.
Transfer chocolate mixture to a large bowl. Place the bowl into a larger bowl filled with ice and water. Gradually add powdered sugar, beating at low speed with an electric mixer. Increase speed to medium-high, and beat 5 minutes or until frosting is cool, thick, and spreadable. Stir in 1 tablespoon vanilla.
Heavenly Candy Bar Cake: Spoon batter into 3 greased and floured 9-inch cakepans. Bake at 350° for 30 minutes or until a wooden pick comes out clean. Cool in pans on a wire rack 10 minutes; remove cakes from pans, and let cool completely on wire rack. Spread half of Chocolate-Marshmallow Frosting evenly between cake layers. Spread remaining frosting evenly over top and sides of cake. Garnish, if desired. Makes 12 servings.