Squares:
Preheat the oven to 325 F (160 C) and place the rack in the center.
Butter an 8-inch (20 cm) square baking dish and line the bottom with parchment paper, leaving an overhang on two sides.

Melt the butter and milk chocolate in a double boiler. Let cool.
Using an electric mixer, beat the eggs with the brown sugar, vanilla, and salt.
At low speed, incorporate the flour and the chocolate mixture.
Using a spatula, fold in 2/3 of the hazelnuts, then pour the batter into the prepared baking dish.
Sprinkle the top of the batter with the remaining hazelnuts.
Bake for 40 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Let cool.

Caramel Sauce:
In a saucepan, bring the sugar and water to a boil.
Cook, without stirring, until the mixture turns golden brown.
Remove from heat and add the heavy cream. Be careful of splatters.
Return to a boil, stirring until the mixture is smooth.
Let cool and drizzle over the cake.

Marshmallow Topping:
In a saucepan, melt the marshmallows with the heavy cream over medium heat, stirring constantly.
Let cool, then pour over the cake.
Serve warm or at room temperature.