• Preheat the oven to 350°F. Grease and flour a 13x9-inch baking pan.
• Combine Coca-Cola, buttermilk, and vanilla extract in a small bowl. Set aside.
• Whisk together flour, cocoa powder, and baking soda in a medium bowl.
• Beat softened butter and sugar in a large bowl on low speed until smooth. Add eggs one at a time, mixing well after each addition.
• Alternately add the flour mixture and Coca-Cola mixture to the butter-sugar mixture, mixing at medium speed until smooth. Scrape down the sides of the bowl as needed.
• Fold in the marshmallows. Pour the batter into the prepared pan.
• Bake for 30–35 minutes, rotating the pan halfway through. A toothpick inserted in the center should come out clean. Cool completely in the pan.

Make the icing

• Heat butter, Coca-Cola, and cocoa powder in a saucepan over medium heat, stirring until the butter melts.
• Whisk in powdered sugar, one cup at a time, until smooth and glossy. Stir in vanilla extract.

Assemble

• Spread the warm icing over the cooled cake. Let it set, and garnish with chopped nuts or additional marshmallows if desired.