Spray 9-inch square pan with cooking spray. In 3-quart saucepan, melt butter over low heat. Add marshmallows; cook and stir until completely melted. Remove from heat.
Stir in hazelnut spread until blended. Add cereal; stir until well coated.
Using buttered spatula or sheet of parchment paper, evenly press mixture in pan. Cool completely, about 45 minutes. For bars, cut into 4 rows by 3 rows.