Prepare a 9x13 baking dish by spraying with non-stick spray or rubbing with butter. Set aside.

In a large pan, melt the butter and marshmallows over low heat until melted, stirring constantly. Be careful not to overheat, as the mixture will burn easily.

Remove from heat, and add pumpkin puree, pumpkin pie spice, and vanilla. Stir to combine.

Add cereal and stir well to combine. Make sure to coat all the cereal in the marshmallow mixture.

Transfer to the prepared baking dish and gently press down with a rubber spatula to flatten and evenly distribute into the dish.

Cool for 30 minutes to an hour or until set. Slice into 12 squares.

Melt white candy melts in the microwave (according to package directions) and then drizzle over the top of the cooled squares.