For the cake: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch glass baking dish with cooking spray.
In a saucepan, melt the butter. Whisk in the cocoa powder, then the boiling water. Let it bubble up for a few seconds, then turn it off and set it aside.
In a large bowl, mix together the flour, granulated sugar and salt. Pour the chocolate mixture over the flour mixture and stir a few times to start to cool the chocolate.
In a separate small bowl, mix the buttermilk, baking soda, vanilla and eggs. Pour over the flour-chocolate mixture and mix until everything is combined. Pour into the prepared baking dish and bake until a toothpick inserted in the center comes out clean, about 20 minutes. Remove from the oven and turn on the broiler.
Sprinkle the marshmallows over the cake, then return the dish to the oven to broil until the marshmallows start to turn golden, about 1 minute. Set aside to cool.
For the icing: Melt the butter in a saucepan, then whisk in the milk and cocoa powder. Whisk in the confectioners' sugar in 4 to 5 additions. Stir in the nuts and vanilla.
Pour the icing all over the marshmallows, allowing some marshmallows to remain visible. Let the icing set, then deliver or serve.