Preheat the oven to 350 degrees F. Spray a 9-by-13-inch disposable aluminum cake pan with baking spray.
Prepare the devil's food cake mix according to the package instructions. Pour the batter into the prepared pan. Bake until the cake springs back when pressed in the center, 20 to 25 minutes. Let cool completely.
With a short end of the cake pan facing you, use a knife to cut a vertical line through the center that divides the cake in half. Fold a 13-inch length of aluminum foil onto itself several times so you have a roughly 3-by-13-inch piece that will fit inside the pan and into the line cut into the cake. This will serve to divide the entree from the side dishes.
Turn the cake so that a long side is facing you. For the mashed potatoes, pipe or spread the vanilla frosting over the right side of the top half of the cake. Press together the lemon candy chews and place in the center for a pat of butter. For the peas and carrots, pile the lime green coated chocolates and mini orange candy chews over the left side of the top half of the cake.
For the Salisbury steak, pour about 3/4 cup caramel sauce over the entire bottom half of the cake. Stack and press 3 marshmallow treats together into one block, then press in the edges and corners with your fingers to round them. Repeat with the remaining treats. Put the cocoa powder in a shallow dish and press the treats into the powder so they are completely coated. Tap off any excess and arrange these Salisbury-steak-lookalikes on top of the cake. Pour a little caramel sauce over each treat for gravy.