Make the marshmallow mixture for the crust: Place a large pot over medium heat and melt butter over gentle heat. Add the mini marshmallows and stir constantly until they are completely melted and smooth.
Add the cereal: Remove the pot from the heat and gently fold the Rice Krispies into the melted marshmallows. Stir just until the rice cereal is evenly coated.
Shape the Rice Krispie crust: Lightly coat a 9-inch pie dish with cooking spray. Transfer the cereal mixture to the dish and press it around the outer edge to resemble a pie crust, leaving the center open for the pumpkin filling. Set it aside while you prepare the next layer.
Begin the pumpkin filling: Using the same pot, melt butter over medium-low heat. Stir in the pumpkin puree and pie spice, mixing until the pumpkin is evenly incorporated.
Melt the marshmallows: Add the remaining mini marshmallows and stir gently and continuously until smooth. Keep the heat moderate; overheating marshmallows is one of the easiest ways to end up with hard Krispies treats.
Add the orange color: Stir in two drops of orange gel food coloring. If you would like a brighter pumpkin color, add one more drop. A little goes a long way.
Add the remaining cereal: Fold the Rice Krispies into the pumpkin-marshmallow mixture until everything is evenly coated. Work fairly quickly, as the mixture will begin to set as it cools.
Fill the “pie.”: Immediately spoon the pumpkin Rice Krispie mixture into the center of your prepared crust. Gently press it into an even layer, making sure the filling touches the crust so the two sections hold together when sliced.
Let the treats set: Allow the pie to rest at room temperature for about 20 minutes. Once set, cut it into wedges just as you would a pumpkin pie. Add whipped topping right before serving, if desired.