3/4 cup chocolate-hazelnut spread, such as Nutella
Line a 13-by-9-inch baking sheet with parchment paper. Lightly grease with 1 tablespoon butter.
Melt the remaining 3 tablespoons butter in a large stockpot or Dutch oven on medium-low heat. Add the mini marshmallows and cook, stirring constantly, until melted, 3 to 5 minutes. Add the puffed rice cereal and gently stir to combine. Pour onto the prepared baking sheet. With clean and lightly oiled hands, press the mixture into the baking sheet. Let cool completely.
Remove the crispy rice bar from the baking sheet in 1 piece and cut in half. Spread the marshmallow creme on 1 of the halves. Spread the chocolate-hazelnut spread on the other. Place both halves on top of each other to create a sandwich. Refrigerate to firm up, about 10 minutes.