To make a toasted marshmallow cake frosting follow the below instructions. Before start preparing get ready with the following ingredients such as: 1 devil's food cake mix, and ingredients to prepare cake, 12 large marshmallows, 1/2 cup chopped walnuts, 1/2 cup firmly-packed light brown sugar. The preparation steps for this delicious toasted marshmallow cake frosting is very easy. First, preheat oven to 350 degrees Fahrenheit and grease a 9 x 13 baking dish. Then Prepare the cake mix according to package directions and pour it into the prepared baking dish. Now cut marshmallows in half and place marshmallows over cake batter, cut side down. Next, sprinkle walnuts and brown sugar over marshmallows. At last, bake on the center rack of the oven for 40-45 minutes until a knife inserted in the center of the cake comes out clean. Note that if marshmallows brown too quickly, cover the cake with aluminum foil to finish baking. If you follow the steps in this video, you can easily make a tasty toasted marshmallow cake frosting.