Preheat oven to low broil and move oven rack to the second from the top (high enough up to broil your marshmallows). Place baking sheet on rack.
Heat milk and water in a saucepan over medium heat until warmed through - about 5 minutes. Be careful not to overheat or scald.
Add cocoa powder, chocolate syrup, sugar, stevia and salt and whisk vigorously until thoroughly combined.
Meanwhile, take a few marshmallows and rub them around the top of your mugs so the graham crackers will stick. Then, dip mug in graham cracker crumbs until well lined. You could also add some graham cracker crumbs to the bottom of your mug for extra graham flavor.
Pour in hot chocolate and top with 1/4 cup marshmallows each. Carefully set mugs on the baking sheet in the oven and broil marshmallows until browned, watching carefully as to not let them burn.
Carefully remove from the oven with a towel and serve with holders or a small towel to protect hand from heat (they should cool quickly for handling). Top with a drizzle of chocolate syrup and extra graham cracker crumbs for serving (optional).
* Adjust flavors before serving, adding more chocolate or sugar if you prefer. * Also, you could add a shot of Kahlua or Bailey's to make it a boozy hot chocolate.