Ingredients

  • Baking spray
  • 2 cups (about 10 ounces) all-purpose flour
  • 1 cup sugar
  • 1 cup packed dark brown sugar
  • 3/4 teaspoon salt
  • 16 tablespoons (2 sticks) unsalted butter
  • 1/4 cup natural cocoa powder
  • 1 cup Coca-Cola
  • 1 teaspoon baking soda
  • 1/2 cup buttermilk, at room temperature
  • 2 large eggs, at room temperature and lightly beaten
  • 1 teaspoon pure vanilla extract
  • 2 cups miniature marshmallows
  • 8 tablespoons (1 stick) unsalted butter, at room temperature
  • 1/4 cup natural cocoa powder
  • 6 tablespoons Coca-Cola
  • 16 ounces confectioners' sugar, sifted
  • 1/4 teaspoon salt
  • 1 teaspoon pure vanilla extract
  • 1 cup pecans, toasted and coarsely chopped (optional)

Method

  • Adjust oven rack to middle position and preheat oven to 350°F. Coat a 13- by 9-inch baking pan with baking spray. Combine flour, sugar, brown sugar, and salt in large bowl.
  • Bring butter, cocoa powder, and Coca-Cola to boil in small saucepan over medium-high heat, stirring occasionally until mixture is homogeneous, and stir into flour mixture.
  • Stir baking soda into buttermilk until completely dissolved and add to flour mixture; whisk to combine. Add eggs and vanilla and whisk to combine. Stir in marshmallows.
  • Scrape batter into prepared pan and bake until tester inserted in center of cake comes out clean, about 45 minutes. Transfer cake to cooling rack and cool 10 minutes, then make icing.
  • Bring butter, cocoa powder, and Coca-Cola to boil in large saucepan over medium-high heat, stirring occasionally. Whisk in sugar, vanilla, salt, and (optional) pecans and pour onto cake immediately, spreading and swirling decoratively with back of a soup spoon. Serve warm, or cool to room temperature.