Preheat oven to 350 degrees F. Grease the bottom and sides of a 9-inch springform pan. In a microwave-safe bowl, heat chocolates and butter together in small bursts, just until melted. Stir until smooth. Sift together flour & baking powder. In a large bowl, whisk together eggs and sugar. Whisk in chocolate mixture. Add the dry ingredients, espresso powder and salt. Stir until combined. Spread brownie batter in the prepared pan. Bake until a toothpick inserted in the center comes out clean, about 15 minutes. Cool completely before continuing with the recipe.
PREPARE THE ICE CREAM LAYER:
Take ice cream out of the freezer and soften for a bit, just until it's easy enough to mash around and spread. Spread the ice cream over the brownie layer.
PREPARE THE MARSHMALLOW LAYER:
In a medium bowl, whisk together marshmallow cream and water. Stir in 3 cups of marshmallows. Spread the marshmallow mixture over the ice cream. Sprinkle 2 remaining cups of marshmallows on top. Freeze until the ice cream is hard, at least 6 hours- but best overnight.
TOASTING THE MARSHMALLOWS:
If you have a handheld blowtorch, run a knife around the inside edge of the springform pan. Release the latch and remove the ring. Use a very sharp, wide, thin knife to run under the brownie layer to release it from the pan and transfer it to a serving platter. Torch the marshmallows until they're golden brown. If using the broiler, adjust the rack so the ice cream cake will be 2 to 3 inches from the heating element. Preheat the broiler. Leave the cake in the pan, put it on a baking pan, and place it under the heating element. Broil until golden brown, about 2 minutes. Run a knife around the inside edge of the pan and remove the pan.
Serve immediately with a good amount of hot fudge. Freeze leftovers and eat as you wish, for up to one week.
*SHORTCUT: use a boxed brownie mix for the base.
Top with crumbled graham crackers to complete the "S'mores" theme, if desired.
Nutritional information was calculated using vanilla ice cream. Hot fudge was not included in the nutritional information.