Melt the Unsalted Butter (3 tablespoon) over low hear in a large pot. Stir in the Canned Pumpkin Purée (1/3 cup) and heat through for about 30 seconds.
Stir in the Marshmallow (10 ounce). When they are almost completely melted, stir in the Pumpkin Pie Spice (1 1/4 teaspoon), Instant Espresso Powder (1/2 teaspoon), and Salt (1/4 teaspoon). Turn off the heat and cool for 20 minutes.
Stir in the Rice Krispies® Cereal (5 1/2 cup). Spray an 8x8 baking dish with nonstick spray. Spread the mixture into the dish and press gently with a greased spatula.
Add Sprinkles (to taste) if you like.
Let the treats set up for 30 minutes, then slice into nine squares and serve!