In a medium saucepan, melt 1 1/2 tbsp. unsalted butter.

Add in 2 cups of the mini marshmallows, and stir until melted.

Remove from the heat and stir in 2 1/2 cups rice krispies cereal.

Press this mixture around the outside edge of a round cake pan.

Next, to the pot, add 2 1/2 tbsp. unsalted butter, and heat until melted.

Add 4 cups mini marshmallows and stir, until melted.

Add in the pumpkin puree and pumpkin pie spice, stirring to combine.

Remove from the heat and stir in 4 cups rice krispies cereal.

Add this mixture to the middle of the cake pan and smooth out evenly. I like to use wax paper or parchment for this.

Allow the rice Krispie treats 1 hour to set, then cut into slices.

Add whipped cream, if desired.