Preheat oven to 350 degrees (F). Spray a 9×13 pan with non-stick spray and line with parchment paper or foil for easy removal once cooled.
In a large bowl whisk the flour, cocoa powder, espresso powder, salt baking soda, and baking powder until well combined and free of lumps. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment or in a large bowl with a hand mixer beat the sugar and vegetable oil until light and fluffy.
Add the eggs one at a time, mixing well and scraping down the sides of the bowl between each addition. Add the vanilla and combine.
Slowly add the milk to the sugar mixture, alternating with the dry ingredients.
Mix until just combined. Scrape down the sides of the bowl to ensure it is fully combined. The batter will be thin.
Pour the batter into the prepared pan. Bake the cake for 30-35 minutes or until a toothpick inserted in the center of the cake comes out with only a few moist crumbs.
Remove the cake from the oven and allow the cake to cool before frosting.
For Marshmallow Icing:
In the bowl of a stand mixer combine the egg whites, sugar, cream of tartar, and salt.
Place the bowl over a saucepan with a couple of inches of simmering water.
Whisk the mixture constantly as it heats up. Whisk for about 6 minutes. The mixture should be warm to the touch and the sugar should be dissolved. If you have a thermometer you can heat the mixture to 115°F.
Transfer the bowl to the stand mixer fitted with the whisk attachment. Beat on high speed until stiff, glossy peaks form. This will take about 10 minutes.
Add the vanilla at the very end of mixing and beat until combined.
For Graham Cracker Crumble:
In a small bowl mix the graham cracker crumbles, melted butter, salt, and sugar until well combined.
S'mores Sheet Cake Assembly:
If desired, remove the chocolate sheet cake from the pan before icing.
Spread the marshmallow icing on top of the chocolate cake. I recommend using a kitchen torch to toast the icing, but you can also place it under the broiler in your oven, just be sure to watch it very closely.
Sprinkle the buttered graham cracker crumbles on top of the toasted marshmallow icing.