Preheat your oven to 350 degrees F. Grease a 9x13 inch baking dish set aside.
Place chopped pecans evenly on a baking sheet and toast for 5-8 minutes. Remove from the oven to cool and use for topping later.
In a large bowl, combine the cake mix, milk, melted butter, and eggs until smooth. (this will replace the ingredients on the back of the cake box) Pour the batter into the prepared baking dish. Bake according to the package directions, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool for about 10 minutes. Then, using the handle of a wooden spoon, poke holes all over the top of the cake, making sure to poke all the way down to the bottom.
In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Remove from heat and stir in the chocolate chips until smooth and well combined.
Pour the ganache over the poked cake, making sure to fill the holes. Sprinkle 10 oz of mini marshmallows evenly over the ganache while it's still warm.
Return the cake to the oven for 1-2 minutes or until the marshmallows are slightly melted. Keep a close eye on it to prevent burning.
Let the cake cool some while making the icing.
In a medium bowl, mix together the melted butter, unsweetened cocoa powder, and heavy cream until combined.
Gradually add the powdered sugar, 1 cup at a time, and beat with a mixer until smooth.
Spread the icing evenly over the cooled cake.
Sprinkle the top of the cake with the toasted, chopped pecans and the reserved ½ cup of mini marshmallows.
Optionally, place the cake under the broiler for an additional 1-2 minutes to lightly toast the marshmallows on top, watching closely to prevent burning.