I got this from a blog called The Kitchn (yes, it is spelled that way):
3 tsp unsalted butter
1/4 cup canned pure pumpkin puree
1-10 oz bag mini marshmallows, plus 1 cup
1/4 tsp pure vanilla extract
1/4 tsp ground cinnamon
pinch freshly grated nutmeg
pinch kosher salt
6 cups puffed rice cereal (I used Erewhon's Crispy Brown Rice Cereal)
Butter a 9x13 baking dish (or a smaller dish for larger treats).
In a heavy saucepan, melt butter over MED-LOW heat. Add pumpkin and continue to cook until it is warmed through. Fold in 10 oz of marshmallows, stirring frequently until almost completely melted. Mix in vanilla, cinnamon, allspice, nutmeg and salt and remove from heat.
Allow the marshmallow mixture to cool 10 minutes; fold in remaining cup of marshmallows. Continue to cool for another 25 minutes until it is room temperature. Failure to cool enough will result in soggy treats. Add puffed rice cereal and stir, using a silicone spatula sprayed with nonstick spray, until combined.
Press the mixture into the greased rectangular baking dish. Let set for 30 minutes before cutting and serving.