Preheat your oven to 325°F. Line an 8-inch square baking pan with nonstick aluminum foil, or regular heavy-duty aluminum foil and spray it with nonstick cooking spray. Set the pan aside.

In a medium-sized bowl, place the flour, xanthan gum, cornstarch, pumpkin pie spice, baking soda, and salt, and whisk to combine well. Set the bowl aside.

In a medium-sized saucepan, place the butter and sugar and melt over medium-low heat, stirring occasionally to prevent burning.

Bring to a gentle simmer, remove the pan from the heat and set it aside to cool until no longer hot to the touch.

You can immerse the bottom of the pan in some ice water to speed the cooling, but take care not to let the water splash up into the pan or to chill too much that the ingredients re-solidify.

In a large bowl with a handheld mixer or in the bowl of a stand mixer fitted with the paddle attachment, place the egg, vanilla, and pumpkin butter, and beat until foamy (about a minute).

Add the cooled butter and sugar mixture, and beat very well until the mixture lightens a bit in color and begins to increase in volume (about 2 minutes).

Add the flour mixture to the bowl, stir until the streaks of flour disappear, then beat briefly to ensure that everything is combined.

The mixture will be thickly pourable and very sticky. Add about 5 ounces of the chocolate chips, and mix until the chips are evenly distributed throughout the batter.

Pour the batter in the prepared pan, and smooth into an even layer with a wet spatula.

Scatter the remaining chocolate chips evenly over the top and press them gently into the batter to help them adhere.

Place the pan in the center of the preheated oven and bake until golden brown on top and a toothpick inserted in the center of the bars comes out clean, or with a few moist crumbs attached (about 40 minutes).

Remove the pan from the oven and allow the bars to cool for at least 10 minutes in the pan before lifting them out by the edges of the foil.

For cleaner slices, chill the bars until firm, then slice into squares with a sharp knife and serve. Wrap leftovers tightly and freeze.