Preheat your oven to 325°F and position the rack at the lowest level. Prepare a 9x13-inch baking dish by spraying it with vegetable oil. Line the dish with a 12x18-inch piece of heavy-duty foil, allowing the edges to hang over the sides for easy removal later, and spray the foil with cooking spray.
In a small bowl, combine flour, baking powder, and salt; set aside. In a large saucepan, melt butter and brown sugar together until the mixture is bubbling and hot. In a separate large bowl, whisk together eggs and vanilla extract. Gradually mix in the hot sugar mixture, followed by the dry ingredients until combined. If you're adding any extras like chocolate chips, make sure to let the batter cool a bit before folding them in.
Pour the batter into the prepared baking dish and smooth the top. Bake for approximately 45 minutes or until the edges are set and the center is slightly firm. Once baked, take the pan out of the oven and allow the blondies to cool for five minutes. Use the foil handles to lift the blondies from the pan and transfer them to a wire rack. Let them cool completely before slicing into 24 squares.
