Heat oven to 350º. Line a 9×13-inch baking dish with foil or parchment, leaving an overhang on the long sides. In a medium bowl, whisk together the flour, pumpkin pie spice, baking soda and salt. Set aside.

In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar together on medium-high speed until fluffy, about 2 minutes. Add the egg and vanilla and beat until combined. Add the pumpkin puree and continue mixing until well combined. On low speed, add the flour mixture and mix just until incorporated. Using a rubber spatula, fold in the white chocolate and 1 cup of the butterscotch chips (and nuts, if using).

Spread the batter evenly in the prepared pan, then sprinkle the remaining butterscotch chips over the top. Bake for 35-40 minutes, until the edges begin to pull away from the sides of the pan and a toothpick inserted in the center comes out with just a few moist crumbs attached. Transfer pan to a wire rack and allow to cool completely before cutting.

Use the foil/parchment handles to lift the dessert from the baking dish. Cut into 24 squares. Store in an airtight container.