Preheat your oven to 350°F. Line a rimmed baking sheet with unbleached parchment paper and set it aside.

In a large bowl, place the flour blend, xanthan gum, baking powder, salt, sugar, and pumpkin pie spice, and whisk to combine well. Add the egg, pumpkin butter and vanilla extract, and mix to combine well. The dough will be thick and sticky. If necessary to bring it together, knead the dough with wet hands until smooth.

Place the dough in the center of the prepared baking sheet, and shape with wet hands into a loaf that is approximately 7 inches long x 3 inches wide x 1 inch thick. Place in the center of the preheated oven and bake until lightly golden brown and firm to the touch, about 20 minutes. Remove from the oven and allow the loaf to cool for at least 10 minutes, or until only slightly warm to the touch. Lower the oven temperature to 300°F.

Slice the loaf in cross-section on the bias into 8 to 10 pieces, each about 1/3 inch thick. Place the pieces back on the prepared baking sheet, flat and spaced about 1 inch apart. Return the baking sheet to the oven and bake for 10 minutes. Flip each of the cookies over on the baking sheet, return to the oven and finish baking until the underside of the cookies is lightly golden brown (about another 10 minutes). Remove from the oven and allow to cool to room temperature before serving. The cookies will crisp as they cool.

Drizzle with melted chocolate, if desired. Store biscotti at room temperature in an airtight container for 2 days. Freeze any leftovers tightly wrapped in a freezer-safe container.