Heat the oven to 350°F (175°C) and line baking sheets with parchment paper. If the pumpkin purée is very wet, spread it over paper towels and gently blot away excess moisture.

Whisk melted, slightly cooled butter with the brown sugar and granulated sugar until smooth. Whisk in the pumpkin, egg yolk, and vanilla extract.

In a separate bowl, whisk together flour, baking soda, baking powder, cinnamon, salt, and nutmeg. Add the dry mixture to the pumpkin mixture and stir only until no dry flour remains.

Fold in the semisweet chocolate chips. Chill the dough for 30-60 minutes if it is soft or sticky.

Scoop approximately 2 tablespoons of dough per cookie onto the prepared baking sheets, leaving about 2 inches between portions. Bake for 10-12 minutes, until the edges are set and the centers still look slightly soft.

Cool the cookies on the baking sheet for 5-10 minutes, then transfer them to a wire rack. Continue with the remaining dough.