2 cups all-purpose flour
2 cups granulated sugar
4 tablespoons unsweetened cocoa
1/2 teaspoon salt
1 cup butter, room temperature, cut into small pieces
1 cup Coca-Cola
2 large eggs
1/2 cup buttermilk
1 teaspoon baking soda
1 teaspoon vanilla extract

Grease and flour a 13 x 9-inch baking pan; set aside. Preheat oven to 350 degrees F.

Stir together the flour, sugar, cocoa, and salt in a large bowl. Bring the butter and Coca-Cola to a boil in a small saucepan. Pour over the flour mixture and stir with a wooden spoon until combined.

Whisk the eggs in a medium bowl until frothy. Add buttermilk, baking soda, and vanilla extract, and whisk until blended. Stir into the batter until incorporated. The batter will be thin. Pour into the prepared pan and bake for 30 minutes or until a cake tester inserted in the center comes out clean. While cake is baking, prepare the frosting.

Frosting
1/2 cup (1 stick) butter
1/3 cup Coca-cola
1 teaspoon instant coffee
2 tablespoons water
1 pound confectioners' sugar, sifted
5 tablespoons unsweetened cocoa
1 cup chopped walnuts, toasted
1 1/2 cups miniature marshmallows

Bring the butter, Coca-Cola, coffee and water to boil in a small saucepan. Stir the powdered sugar and cocoa together in a medium bowl and pour the butter-Coca-Cola mixture over; stir with a wooden spoon until well blended. Fold in the walnuts and marshmallows. Remove the cake from the oven and pour the frosting over the top of the hot cake, spreading to the sides of the pan. Cool to room temperature. Cake may be kept tightly covered at room temperature up to one week or it may be frozen. Defrost at room temperature.