Mississippi Mud Cake is one of my favorite southern sheet cakes. It's a rich and moist chocolate cake covered with gooey marshmallows, crunchy pecans, and a fudge icicng. Dessert doesn't get any better than this.
Preheat oven to 350 degrees and spray a 9x13-inch pan with baking spray.
Whisk together flour, granulated sugar, brown sugar, cocoa powder, baking powder, baking soda, and salt in a large mixing bowl.
Add oil, eggs, vanilla extract, whole milk, and sour cream. Use a hand-held mixer to mix until combined.
Add boiling water and beat on high for 1 minute. Pour batter into prepared pan. Bake for 30 minutes or until the top feels firm when you gently press down on top.
Sprinkle marshmallows on top. Return to oven for 2 minutes. Let cake cool.
To make icing, combine butter, cocoa powder, and milk in a large saucepan. Heat over medium heat until butter is melted. Stir and cook for 1 minute.
Whisk in powdered sugar 1 cup at a time. Whisk in vanilla extract.
Slowly pour icing over cooled cake. Sprinkle with pecans.