A rich, old-fashioned chocolate sheet cake made in a 9x13 with mini marshmallows and warm chocolate frosting poured right over the top. The Coca-Cola keeps it moist and adds a soft caramel note under all that chocolate. It's easy to make and feeds a crowd, which makes it a potluck and family-dinner favorite.
Preheat the oven to 350°F and grease a 9x13-inch baking dish.
Whisk together the flour, sugar, salt, and baking soda in a large bowl.
In a medium saucepan over medium heat, combine the Coca-Cola, butter, and cocoa powder. Heat until the butter melts and the mixture just begins to simmer, whisking occasionally.
Pour the hot cocoa mixture into the dry ingredients and whisk until combined.
Add the buttermilk, eggs, and vanilla. Stir just until smooth.
Fold in the mini marshmallows.
Pour the batter into the prepared dish and bake for 30 to 35 minutes, until a toothpick comes out with a few moist crumbs.
Chocolate Frosting
About 5 minutes before the cake is done, melt the butter with the cocoa powder and Coca-Cola in a saucepan over medium heat until smooth and hot.
Remove from the heat and whisk in the vanilla and powdered sugar until smooth.
Stir in the nuts if using.
Pour the warm frosting over the warm cake and spread evenly.
Let the frosting set before slicing.
Notes
Frost the cake while both the cake and frosting are warm for the smoothest top.
Have the frosting ready the moment the cake comes out of the oven.
Bake just until a toothpick shows a few moist crumbs so the cake stays moist.
Leave the nuts out for the softest texture, or add them for crunch.