Spray an 11x7 inch pan with nonstick cooking spray and set aside.
In a medium saucepan over medium heat, melt the butter, stirring occasionally with a wooden spoon. Once melted, add the pumpkin puree and pumpkin pie spice and mix well. Add the marshmallows and stir continuously until they're all melted and mixture is sticky, but smooth, about two minutes. Add the cereal and mix well, making sure everything is coated with the marshmallow mixture.
Pour mixture into prepared pan and press firmly down with a rubber spatula spray with cooking spray to avoid sticking. Allow to set for 15 minutes.
In a microwave safe bowl, add the white chocolate chips and place on high for 30 seconds, remove and stir and return to microwave in 30 second increments until melted, stirring between each one. Transfer melted chocolate to a pastry bag or plastic bag with 1/4 and inch of the tip cut off and pipe melted chocolate on top of cereal bars back and forth horizontally, saving a little extra to add the eyes. Place bars in the fridge for 10 minutes to set.
Cut bars in long rectangles and use left over melted chocolate to place eyes on the bars towards the top of the rectangles.