Preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper.
Add the butter to a small saute pan or saucepan. Set the butter over medium heat, stirring occasionally until the butter bubbles up, then turns amber in color (this took me ~2 minutes.)
Pour the brown butter into a small mixing bowl and set it aside. You should be left with 22g of brown butter.**
If you're using a chocolate baking bar, chop it up into small pieces while the butter is cooling down. Also, place the pumpkin puree on to a napkin and press down on it with the napkin to remove excess moisture, until you are left with only 18g of pumpkin puree.
Add the dark brown sugar, granulated sugar, pumpkin puree, and vanilla into the browned butter and mix until well combined.
Add in the flour, baking soda, and salt. Mix together just until a few flour streaks remain, then add in ~¾ of your chocolate and mix just until no flour streaks remain. Place little scattered dollops of the marshmallow fluff on top of the cookie dough.
Set the cookie dough in the freezer to chill for 5 minutes.
Scoop out all the cookie dough and roll it into one large ball. Top the cookie with extra marshmallow fluff, and your remaining chopped chocolate. Bake for 14-16 minutes, until the edges are golden brown and the middle of the cookie no longer looks “wet.”
Once the cookie comes out of the oven, use a large bowl to gently form it back into a perfect circle. Allow the cookie to rest on the hot baking sheet for 5-10 minutes before transferring it to a wire rack to cool (or just before enjoying it warm.) Serve and enjoy!