Brown the butter: place the butter in a light colored small saucepan set over medium heat. Cook, as it melts, then begins to sputter and spit. You’ll see tiny panko like bits at the bottom of the pan. Keep stirring and cooking, the bits will turn an amber brown then a toasty medium brown. Once they are all brown, immediately transfer to a medium sized bowl.
Add the pumpkin spice (if using) to the butter and stir. Let the butter cool for about 5 minutes.
Add the salt and vanilla and whisk in the sugars. The mix won’t change much, yet.
Whisk in the yolk and pumpkin butter, keep whisking until the mixture is shiny and cohesive.
Stir in the baking powder, soda and flour until half of the flour is mixed in. Then add the chocolate chips and stir until no flour bits are visible.
Line a greased cookie sheet with parchment paper. Divide the dough into four equal pieces and shape each piece into a giant ball. If you like, add more chocolate chips all over the dough (except the bottom). The balls will feel greasy at this point and the dough will be soft.
Let the cookie dough rest uncovered on the pan until firm, for about 15-20 minutes. They won't feel soft or greasy anymore.
Preheat the oven to 375 F. Bake the cookies for five minutes, then lower the temperature to 350 F and bake until golden on the edges, about 12 minutes. Let the cookies cool on the pan (they will be very soft at first). Once they are firm they can be lifted off the pan to serve.