Place butter in a microwave safe dish and melt butter in microwave. This takes about 45 seconds.
Place butter and marshmallows in a large microwave safe bowl. Stir to coat and microwave for 90 seconds. Remove from microwave and stir to ensure that the mixture is smooth. Add pumpkin spice seasoning.
Gently stir cereal into the marshmallows until all coated and evenly mixed.
Spread the rice krispie mixture into a 9x13 pan. Press the mixture into the pan. Tip: use a flat bottomed glass coated lightly with butter to press into pan and it will not stick and allow you to get a flat top. Allow the mixture to cool uncovered for about 10 minutes. Cut the treats into squares. If you find the treats too sticky give them a little more time to cool and set.
Top the rice krispie treats with green frosting and a candy pumpkins