Grease a silicone mold or line a baking sheet with parchment paper to shape the treats.
In a large saucepan, melt the butter over low heat, producing a gentle bubbling sound. Add the marshmallows and stir constantly until completely melted and smooth, creating a sticky, marshmallowy mixture.
Stir in the pumpkin puree along with ground cinnamon and nutmeg, mixing well until the mixture is fragrant and evenly combined, with a creamy orange hue.
Remove the saucepan from heat and gradually fold in the rice cereal, stirring until the cereal is well coated with the pumpkin-spiced marshmallow mixture and looks glossy.
Quickly scoop the mixture into the prepared mold or onto the parchment-lined sheet, pressing down gently with a spatula or your fingers to create an even layer or shape into small mounds.
While still warm, shape the mixture into small pumpkin-like rounds or press into a mold to give the treats a rounded form. Let them cool and set at room temperature for about 15 minutes.
Once cooled, use a small knife or toothpick to decorate the tops of each treat with mini chocolate chips or candy eyes to resemble pumpkin faces or add charming details.
Serve the pumpkin Rice Krispie treats at room temperature, enjoying their crunchy exterior and soft, sweet inside with a festive appearance.
