Grease a 9x13 inch pan generously with butter or nonstick spray. Line it with parchment if you want to lift the slab out cleanly for cutting. Set it near the stove so you are ready to move fast once the marshmallows are melted.

Melt the butter in a large heavy-bottomed pot over medium-low heat. Once it is melted and foamy, add the marshmallows. Stir constantly with a rubber spatula until they are completely melted and smooth, about 4 to 5 minutes. Keep the heat low so the mixture stays pale and does not scorch.

Remove the pot from the heat. Stir in the pumpkin puree, pumpkin pie spice, vanilla, and salt. Mix until the color is even throughout. The mixture will turn a soft orange-tan. It may look slightly lumpy at first, just keep stirring and it will smooth out.

Add the Rice Krispies all at once and fold them in with the spatula until every piece is coated. Work fairly quickly because the mixture starts to firm up as it cools.

Scrape the mixture into the prepared pan. Butter your hands well and press the mixture into an even layer, getting into the corners. Buttered hands grip and press without sticking, and you get a more even layer than with a spatula. Press firmly but do not compact it so hard that the treats become dense.

Let the pan sit at room temperature for at least 30 minutes to set. Do not refrigerate to speed this up or the edges will get hard while the center stays soft.

If you want the white chocolate drizzle, melt the white chocolate chips in a microwave-safe bowl in 30-second intervals, stirring between each, until smooth. Drizzle over the set treats using a spoon or a small zip bag with one corner snipped off. Let it set for 10 minutes before cutting into 16 squares.