In a large pot over medium heat, melt the butter, chocolate hazelnut spread, and marshmallows until smooth, about 5 minutes.
Immediately stir in the crispy rice cereal.
Spread mixture into an 8 or 9-inch pan that's been greased with butter or nonstick spray. Let sit about 30 minutes until firm.
If desired, warm some chocolate hazelnut spread in the microwave and use a fork to drizzle it over the bars. Add flaked or coarse salt.
Slice and serve!
