Step 1

For the cake: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch glass baking dish with cooking spray.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

Step 6

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.

Step 7

Pour the icing all over the marshmallows, allowing some marshmallows to remain visible. Let the icing set, then deliver or serve.