In a medium saucepan, melt 3 tablespoons of butter over medium heat. Once fully melted, add 4 cups of mini marshmallows, stirring constantly until the mixture becomes smooth and completely melted, about 5 minutes. Remove from heat and fold in 6 cups of Rice Krispies cereal, ensuring every piece is well-coated. Allow the mixture to cool slightly until it's warm to the touch but pliable.

Lay a large sheet of parchment paper on a clean surface. Using wet hands or a spatula, evenly spread the Rice Krispies mixture into a rectangle about 12x8 inches. Gently drizzle 1 cup of marshmallow fluff down the center, and sprinkle 1 cup of crushed graham crackers on top. Finally, drizzle a generous amount of chocolate or Nutella over this layer to create a sweet filling.

Use the parchment paper to lift one edge of the Rice Krispies mixture, rolling it away from you to form a tight log, similar to sushi. Make sure to roll carefully to keep the filling intact, using the parchment to guide the roll. Once rolled, use a sharp knife to slice the sushi into bite-sized pieces, approximately 1 inch thick.

Arrange the sliced S’mores Sushi pieces on a colorful serving plate or a wooden board for a delightful display. This fun no-bake dessert can be served immediately or kept in an airtight container for up to three days.