First of all, the folks in Mississippi will tell you that you’re looking for a Mississippi Mud Cake. Second, we’ve looked through dozens and dozens of recipes for "mud cake," and have yet to find any agreement at all among them (one includes crushed pineapple and canned cherry pie filling for heaven’s sake!).
Given that level of controversy, we only feel comfortable stating that a mud cake is a dessert from the American South that includes chocolate. In color, it is supposed to resemble the dark brown-black mud of the Mississippi River. Nowadays, it is a chocolate sheet cake studded with pecans and marshmallows and covered in a dark chocolate icing. In earlier times, it was lighter and more delicate, without nuts, certainly without marshmallows, but with a modest mocha flavor.
Here are choices for each:
Traditional Mud Cake
Preheat the oven to 325°F (160°C). Butter a 10-inch Bundt pan and dust it with cocoa. Mix 3 tablespoons of unsweetened cocoa powder with 2-1/4 cups of all-purpose flour, 1 teaspoon of baking soda and 1/4 teaspoon of salt. In a small sauce pan, dissolve 2 tablespoons of instant coffee in 3/4 cup of boiling water, then add 1/4 cup of coffee liqueur, bring it to a simmer, and then reduce it to low heat. Stir in 5 ounces of unsweetened chocolate and 1 cup of unsalted butter, and stir until melted and smooth. Take it off the heat and stir in 2 cups of sugar, then let it cool to room temperature. Beat three eggs - one at a time - into the coffee mixture, then add the flour mixture in three or four batches, and mix until just smooth. The batter will be very thin.
Pour the batter into the pan and bake until a toothpick or cake tester inserted near the center comes out clean, which should take 60 to 70 minutes. Cool it completely before frosting. To make the glaze, combine 8 ounces of semisweet chocolate, 4 tablespoons of butter and 1 cup of heavy or double cream in a saucepan over low heat until melted and smooth. Allow it to cool for 15 minutes or so, until thick but still pourable. Place the cake on a cooling rack over a cookie sheet and pour the glaze over the entire cake, allowing it to fill every crevice and indentation.
Marshmallow-Pecan Mud Cake
Preheat the oven to 350°F (175°C). In large bowl, beat 1/2 cup of butter, 1 cup of sugar and 1 teaspoon of vanilla until creamy. Add 3 eggs, one at a time, beating well after each addition. Stir together 3/4 cup of all-purpose flour, 1 teaspoon of cocoa powder, 1/2 teaspoon of baking powder and a pinch of salt and then blend it into the butter mixture. Stir in a cup of chopped pecans. Spoon the batter into a greased 13-by-9-by-2-inch baking pan. Bake for 15 to 18 minutes or until the top springs back when touched lightly in center.
While the cake it baking, make the frosting. Beat 6 tablespoons of softened butter, then blend in 2-2/3 cups of powdered sugar, 1/2 cup of cocoa powder and 1/3 cup of milk. Beat to a spreading consistency (additional milk may be needed). Blend in 1 teaspoon of vanilla extract.
Remove the cake from oven; immediately cover the top with marshmallows. Return the cake to the oven for 2 to 3 minutes or until marshmallows are soft. As soon as it comes out of the oven again, spread the frosting over the marshmallow layer. Cool thoroughly before cutting the cake into squares.
Having seen a photo of one of your amazing decorated cakes, we don’t think either version is going to be a very good candidate for fancy decorating.