In a large pot melt your butter on medium low heat. Once the butter is melted add in the marshmallows and stir until they are melted. Stir in about 3/4 of the pumpkin pudding mix, remove from heat.
Gently stir in your Rice Krispies until they are well combined.
Spray a 9 x 13 baking pan with the cooking spray and press the Rice Krispie mixture into the pan. Allow the Rice Krispies to cool.