Prepare the chocolate-hazelnut hot chocolate:
Pour 1 cup of milk and 3 tablespoons chocolate-hazelnut spread into a saucepan. Bring to a gentle simmer over medium heat. Whisk until chocolate-hazelnut spread is melted. Add in the rest of the milk (1 cup), and gently heat till warm. Turn down the heat to a gentle simmer or warm.
Prepare the grilled cheese:
Using the softened butter (1 tablespoon per slice of bread), butter both sides of two pieces of bread. For the other two pieces, butter only one side. Place marshmallows (1/2 cup per slice) on the two pieces of bread that were buttered on both sides. Use the single-sided buttered bread to top off the sandwiches.
Grill at low heat for 5-7 minutes, or until a beautiful brown crust appears. Flip to brown the other side (about 2 minutes). Remove from heat.
Cut each sandwich into fours.
Pour chocolate-hazelnut hot chocolate into little glasses and top off with grilled “cheese.” Serve. (When eating, make sure to dip the sandwiches into the hot chocolate.)