Butter a 9-by-13-inch baking pan and line with parchment paper, leaving about 2 inches of overhang.
Melt 1/2 stick butter in a large pot over medium heat. Add 1 ounce of marshmallows and cook, stirring, until melted.
Remove from the heat and stir in 6 cups of cereal until combined fully.
Firmly press the mixture into the prepared pan and set aside.
Then Melt 1 stick butter in a clean pot over medium heat.
Add in the remaining 20 ounces of marshmallows and stir, until melted.
Then, stir in the allspice and canned pumpkin until blended.
Remove from the heat and stir in the remaining 12 cups cereal.
Firmly press the pumpkin mixture on top of the plain mixture in the pan.
Let cool completely.
Use the parchment overhang to lift the treats out of the pan.
Cut into squares and serve!