In a large pot, melt the butter over medium heat. Reduce the heat to low, then remove 1 tablespoon of the butter and pour it into an 11x7-inch pan; brush the butter edge to edge to grease the pan.

Add the pumpkin puree to the melted butter and cook on low, stirring constantly, for about 1 minute until the pumpkin is hot.

Stir in the marshmallow and cook on low, stirring constantly, until the marshmallows are fully melted. Remove from the heat and mix in the pumpkin spice and vanilla extract.

In a large mixing bowl, combine the Rice Krispies with the pumpkin marshmallow mixture, then scoop into the prepared 11x7-inch pan and use a buttered spatula to spread out evenly.

Chill the Rice Krispies treats in the refrigerator for 1 hour, or until fully set.

Remove the Rice Krispies treats from the pan and place on a cutting board. Use a sharp knife to cut into 12 squares.

Melt the white chocolate according to the package directions. Use a spoon to drizzle the melted chocolate over the Rice Krispies treats. Allow the chocolate to set before serving, which will take about 30 minutes depending on the temperature of your kitchen.