Preheat oven to 350 degrees and spray a 9x13 baking dish with pam baking spray
Using a medium heatproof bowl, melt the butter and cocoa powder
together in increments of 45 seconds until completely melted
Using a large bowl, whisk together the sugar, vanilla and eggs until combined
Gradually whisk in the chocolate butter mixture until combined
In a medium bowl, whisk together flour, salt and gradually whisk into the wet ingredients
Fold in the chopped walnuts
Pour the batter into the baking dish
Bake in the oven for 35-45 minutes or until toothpick comes out clean
As soon as the cake comes out of the oven top with the marshmallows
Place back into the oven for 4 minutes so that the marshmallow starts to slightly melt
Place the cake onto the counter
Pour frosting over marshmallow cream and spread evenly on top. Let chill in fridge for about 30 minutes.
Allow the frosting to set for about an hour before serving!