Start by browning the butter. You'll take ½ cup of butter (one stick) and melt in a medium saucepan, and then continue to cook past the point of boiling. You will do this over medium heat, past the point of the butter foaming, until it turns a nice golden brown, and has a nutty smell. Make sure you are stirring and watching it, because it will turn from perfectly brown to burnt fairly quickly. Once finished browning, pour all of it into a bowl and allow to cool completely.
Preheat oven to 350 F. Line two baking sheets with parchment paper.
Measure out ½ cup of pumpkin and place on a paper plate. Using some paper towels, blot the pumpkin, removing liquid until the paper towel wipes clean. When done the pumpkin should measure more like ⅓ cup (80g).
Place browned butter and sugars in the bowl of a stand mixer, fitted with a paddle attachment and mix together. Add pumpkin, egg yolk and vanilla and mix again.
In a separate bowl, whisk together flour, baking soda, salt, pumpkin pie spice and cinnamon. Add flour mixture to bowl and mix on low, being careful not to overmix.
In a small blender or food processor pulse the oats 4-5 times until they become a mixture of oats and oat flour. Add this to your cookie dough and mix again. Fold in butterscotch chips.
Scoop the dough into large balls, I like to use a ⅓ cup cookie scoop. Top with a few extra butterscotch chips and sprinkle with a bit of flaked salt if desired.
Bake the cookies for 11-14 minutes. You don't want to see any shiny dough in the middle of the cookie. Allow cookies to cool on the pan before moving to a cooling rack to hold their shape.