Preheat the oven to 350F. Line two baking sheets with parchment paper and set aside.
In a medium bowl, whisk together the gluten-free flour blend, xanthan gum (if using), baking soda, salt, and pumpkin pie spice. Set aside.
In a large bowl, mix together melted butter, pumpkin, sugar, brown sugar and vanilla. Then gradually add in the dry ingredients to the sugars, mixing thoroughly. Mix until completely combined.
Stir in the chopped chocolate, reserving about 1 tablespoon for the top.
Spoon 1.5 tablespoon sized balls of dough onto parchment lined baking sheets, leaving about 2 inches in between each dough ball. I typically use 2 baking sheets.
Press down slightly on the balls of dough as they won't spread very much during baking. Bake at 350F for 8-10 minutes or until edges and bottom of cookies are slightly browned.
Immediately, press the remaining chopped chocolate into the dough balls and sprinkle with flaky sea salt, if using.
Let cool on the baking sheet for 5 minutes before transferring to a wire cooling rack.