Preheat the oven to 350°F. Grease an 8×8-inch baking pan or line it with parchment paper.
In a large mixing bowl, whisk together the melted butter and brown sugar until combined.
Add the egg and vanilla extract, whisking until smooth.
Stir in the pumpkin purée until thoroughly combined.
In a separate bowl, whisk together the flour, baking powder, and pumpkin pie spice.
Gently fold the dry ingredients into the wet ingredients just until no dry streaks remain.
Fold in the chocolate chips.
Spread the batter evenly into the prepared baking pan.
Bake for 20–28 minutes, until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs rather than wet batter.
Allow the blondies to cool completely before slicing into 9–12 bars. For cleaner slices, refrigerate for about 20 minutes before cutting.