Pour 5 cups Rice Krispies cereal into a large mixing bowl. Set aside. Lightly grease an 8x8-inch pan with nonstick spray or butter.
In a medium saucepan over low heat, melt 1/4 cup salted butter. Once melted, add 4 cups mini marshmallows. Stir constantly until the marshmallows are fully melted and the mixture is smooth, about 3–5 minutes.
Add in 1/2 cup chocolate hazelnut spread and stir until smooth, then remove the saucepan from heat.
Immediately pour the melted marshmallow mixture over the cereal. Stir with a rubber spatula until the cereal is evenly coated.
Transfer the mixture to the prepared 8x8-inch pan. Using a piece of wax or parchment paper, press the mixture evenly into the pan. Be careful not to press too hard to keep the treats light and chewy.
Allow the treats to cool completely at room temperature, about 20–30 minutes. For faster cooling, place the pan in the refrigerator for about 5 minutes.