In a medium saucepan over medium heat, melt the butter. Continue cooking, stirring often, until the butter foams, then turns golden with brown specks at the bottom and smells nutty (about 5–7 minutes). Immediately pour into a large mixing bowl to cool slightly.
Once the butter has cooled for about 5 minutes, whisk in the brown sugar and granulated sugar until smooth. Stir in the pumpkin puree, egg yolk, and vanilla extract until fully combined.
In a separate bowl, whisk together the flour, oats, baking soda, baking powder, cinnamon, pumpkin pie spice, and salt.
Add the dry ingredients to the wet and stir until just combined. Fold in the chocolate chips.
Cover the bowl and refrigerate for at least 30 minutes (or up to 24 hours) to let the flavors develop and firm up the dough.
Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper. Scoop about 2 tablespoons of dough per cookie, spacing them 2 inches apart. Bake for 11–13 minutes, or until the edges are set and lightly golden but the centers still look soft.
Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack. Sprinkle with flaky sea salt, if desired.
