Preheat the oven to 350°F and line a 9x9 pan with parchment. Blot the pumpkin puree with paper towels to remove excess moisture.
Whisk the melted butter and both sugars until smooth. Whisk in the egg, yolk, pumpkin, and vanilla.
Add the flour, pumpkin pie spice, baking powder, and salt and stir just until combined. Fold in most of the chocolate chips.
Spread the thick batter into the pan and scatter the reserved chips on top. Bake for 28 to 32 minutes, until the edges are set and the center still looks slightly underdone.
Cool completely in the pan before cutting into squares.
Notes
Blot the pumpkin: Removing excess water is the key to chewy, not cakey, blondies.
Melted butter: Keeps the bars dense and chewy.
Underbake slightly: Pull them when the center still looks soft.