Stabilize the grill at 350°F and set up for an indirect cook.
In a large bowl, combine flour and granulated sugar.
In a small saucepan, melt 2 cups of butter over medium heat. Whisk in ½ teaspoon salt and ¼ cup of the cocoa, until well combined. Stir in 1 cup of boiling water.
In a large bowl, whisk together buttermilk, beaten eggs, baking soda, and 1 teaspoon vanilla.
Combine the mixtures created in steps 3 and 4, whipping until fully incorporated. To make the batter, slowly pour and whip the liquid mixture into the dry mixture in step 2. Be careful not to overmix.
Grease a half sheet tray with butter (18x13- inch or similar) and dust with flour. Then, pour the batter into the tray and ensure it spreads evenly over the pan.
Bake on the indirect grill for 20 to 25 minutes. During the last 5 minutes of the cook, add ¾ of the marshmallows and macadamia nuts to allow them to pick up some smoke and color. Remove and cool for 10 minutes.
To make the icing, heat a saucepan and melt the remaining 1¾ cups of butter. Take off the heat and add remaining ¼ cup cocoa powder, whole milk, remaining 1 teaspoon vanilla, confectioners’ sugar, and remaining ¼ teaspoon salt. Work quickly so icing stays fluid.
Drizzle frosting over slightly cooled cake (reserve a bit for plate presentation). Refrigerate for later service or allow frosting to set for 10 minutes or so and cut one piece out and transfer it to the presentation plate. Cut another piece out and place it on top of the 1st piece. It’s all about the layers!
Garnish the plate with a bit of frosting drizzle and any leftover marshmallows. Enjoy!