Heat oven to 350°F; grease a 9x13-inch pan. Whisk flour and sugar in a large bowl.
Bring 1 cup butter, 3 tablespoons cocoa, and 1 cup Coca-Cola to a boil in a saucepan over medium heat; stir into flour mixture.
Mix in buttermilk, eggs, baking soda, vanilla, and marshmallows until combined and marshmallows evenly distributed.
Pour into pan; bake 30-35 minutes until a toothpick in center comes out clean.
About 5 minutes before cake is done, bring 1/2 cup butter, 3 tablespoons cocoa, and 6 tablespoons Coca-Cola to a boil for frosting.
Remove from heat; stir in powdered sugar and pecans until smooth.
Spread warm frosting evenly over entire hot cake immediately after baking.
Cool completely before cutting and serving.