In the bowl of a stand mixer, whisk the egg whites until stiff peaks form, 5 minutes. Transfer to a medium bowl, cover with plastic wrap and store in the refrigerator until needed.
Preheat the oven to 320º and line a 9-inch-square cake pan with parchment paper. In the same bowl of the stand mixer, add the egg yolks and sugar, and cream until light and fluffy, 3 minutes. Add the butter, cocoa powder, espresso powder, salt and vanilla, and mix until everything is fully incorporated. Add the flour and mix just until the flour is mixed into the other ingredients. While the mixer is running on low, stream in the milk until it is all incorporated. Fold in the whipped egg whites and pour the batter into the prepared pan. Bake until the cake no longer jiggles when shaken, 40 to 45 minutes. Remove from the oven and allow to fully cool.
Garnish with whipped cream, mini marshmallows and chocolate syrup, then dust with hot chocolate powder and serve.