In a medium large pot, melt butter over medium heat. Add mini marshmallows and stir until marshmallows become a smooth cream. Remove from oven. Melt 6 ounces of Nutella in the microwave for 20 seconds or until it’s pourable. Add to the marshmallow cream and stir until well combined. Working quickly, add Rice Krispies and stir until completely coated with marshmallow Nutella cream. Spray an 8 x 8 inch baking tin with cooking spray. Press Rice Krispies firmly into pan in an even layer. Melt 8 ounces of Nutella in microwave for 20 seconds. Melt white chocolate in microwave at 30 second intervals until completely melted. Pour the Nutella and white chocolate over the top of the Rice Krispies. With a skewer, knife or chop stick, move the chocolates to create a swirling effect. Refrigerate for 1-2 hours. Cut into squares and enjoy!