Spray a 9 X 13 inch baking pan with cooking spray and set aside.

Heat the butter in a large nonstick saucepan over low heat until melted.

Add the marshmallows and Nutella.

Continue cooking, stirring until smooth, 2 to 3 minutes.

Remove the pan from the heat.

Add the Cheerios, and stir until well mixed.

Press the mixture evenly into the prepared pan.

Cool completely until set, about 20 minutes, before cutting into squares.