Choose your desired size baking pan, I used a square 8x8x2, and spray generously with nonstick cooking spray. Set aside.
In a large pot, melt butter or margarine over medium heat. Stir in pumpkin puree and cook until warmed through.
Fold in marshmallows, stirring frequently until melted. Remove from heat and stir in vanilla, and spices
Let marshmallow mixture cool at room temperature for 20 minutes (cooling helps avoid soggy treats). Add the crispy rice cereal and stir until combined.
Pour mixture into prepared baking pan, spread out evenly and gently press down with a rubber spatula.
Refrigerate to set for at least 1 hour. Cut into squares
Store in an airtight container up to one week.