Heat the oven to 350°F. Line a 9-by-13-inch pan with parchment, leaving overhang for lifting.
Whisk flour, baking powder, baking soda, cinnamon, nutmeg, and salt in a medium bowl.
In a large bowl, whisk melted butter and brown sugar until smooth. Whisk in pumpkin, eggs, and vanilla.
Fold the dry ingredients into the pumpkin mixture just until no dry streaks remain, then fold in the chocolate chips.
Spread batter evenly in the prepared pan and sprinkle with extra chocolate chips if using.
Bake 23 to 27 minutes, until the center is set and a toothpick comes out with a few moist crumbs.
Cool completely in the pan, then lift out with the parchment and cut into 24 bars.