Preheat oven to 350 F. Grease a 9x13-inch baking pan. Set aside.
Combine butter, cocoa powder, and cola in a medium saucepan over medium heat, stirring until butter melts.
Stir in marshmallow, and heat until mostly melted.
Remove from heat.
Stir in flour, sugar, and baking soda.
Add eggs, and stir until blended.
Stir in buttermilk, Greek yogurt, and vanilla.
Pour into prepared baking pan.
Bake for 20-30 minutes, or until a pick inserted in center comes out clean.
Make the icing during the last 10 minutes of baking.
To make the frosting, combine butter, cocoa, and cola in a medium saucepan over medium heat, stirring until butter melts.
Remove from heat, and stir in powdered sugar, vanilla, and pecans.
Remove cake from oven, and carefully pour icing over warm cake.
Notes
Cola: You'll need almost an entire 12 ounce bottle of coca cola. Also, I recommend using regular, not diet, cola.
Greek yogurt: If you don't have Greek yogurt, you can use all buttermilk in this recipe (for a total of 3/4 cup). To make a quick buttermilk substitute, you can use this recipe.
This cake tastes best after sitting for at least an hour. I like to make it either earlier in the day or the day before I plan to serve it.