Adjust oven rack to lowest position and heat oven to 325 degrees. Spray an 8-inch square metal baking pan with vegetable-oil cooking spray. Fold a 16-inch length of heavy-duty foil to 7 1/2 inches wide and lay it across pan bottom and up two sides so you have foil overhangs to pull blondies from pan. Spray foil with vegetable-oil spray.

Whisk flour, baking powder, cinnamon, and salt in a medium bowl. Whisk brown sugar into butter in a medium bowl. Whisk eggs and vanilla together in a small bowl, then whisk into butter mixture. Whisk in dry ingredients until just combined. Fold in cinnamon chips or stir-in of your choice. Pour batter into prepared pan. Bake until blondies are just set, about 40 minutes. Remove from oven and let cool for 5 minutes. Using foil handles, pull blondies from pan, set on wire rack, and cool to room temperature. Cut into squares and serve.