Preheat your oven to 350°F and grease a 9x13-inch baking pan. In a large bowl, whisk together the flour and sugar.
In a saucepan over medium heat, combine 1 cup butter, 3 tablespoons cocoa powder, and 1 cup Coca-Cola. Bring to a boil, then pour over the flour mixture and stir well.
Add the buttermilk, beaten eggs, baking soda, vanilla extract, and marshmallows to the batter, mixing until well combined and marshmallows are distributed throughout.
Pour the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
About 5 minutes before the cake is done, prepare the frosting by combining 1/2 cup butter, 3 tablespoons cocoa powder, and 6 tablespoons Coca-Cola in a saucepan and bringing to a boil.
Remove frosting from heat and stir in the powdered sugar and chopped pecans until smooth.
Pour the warm frosting over the hot cake as soon as it comes out of the oven, spreading evenly to cover the entire surface.
Allow the cake to cool completely before cutting and serving.