Preheat the oven to 350°F / 175°C. Line a baking sheet with parchment paper or with a silicone mat.
In a small saucepan set over medium heat, brown the butter. Stir the butter occasionally until it turns amber in color and develops little brown specks. At this point, weigh your brown butter to make sure you have at least 90g. If you have a little less, mix in some water until you reach 90g.
Transfer the brown butter to a medium sized mixing bowl and set it in the freezer for 5 minutes to chill. In the meantime, chop up your chocolate bar if using one and prep the pumpkin puree.
Use a napkin or cheesecloth to gently strain water out of the pumpkin puree. (You may need multiple napkins) Continue to squeeze/pat water out of the puree until you are left with only 60g of pumpkin puree.
Once the butter has cooled, whisk the pumpkin puree, granulated sugar, and dark brown sugar into it.
Whisk in the egg and vanilla until well combined.
Add in the flour, oats, baking soda, baking powder, salt, and pumpkin pie spice. Use a rubber spatula to fold the mixture together until a few flour streaks remain.
Add in the chopped chocolate and fold just until no flour streaks remain.
Scoop out heaping 3 tbsp. scoops of dough, setting them on the cookie sheet at least 2" apart to allow room for spreading.
Top each cookie with additional chocolate (optional - but recommended!)
Bake for 12-14 minutes, until the edges of the cookie have set.
Immediately after taking the cookies out of the oven, you can take a bowl that is slightly larger than the cookies and swirl it around over them to make them a little more uniform and circular.
Allow the cookies to rest on the hot baking sheet for at least 5 minutes before transferring to a wire rack to cool. Enjoy!