Preheat the oven to 350°F (175°C). Line a greased 9" x 5" loaf pan with parchment paper, and grease the paper. Place the lining so that it overhangs the edges of the pan to create a sling for lifting the blondies once they're baked.
Whisk together the flour, baking powder, and salt. Set aside.
In a medium bowl, combine the cooled butter, brown sugar, egg, and vanilla.
Gradually stir the flour mixture into the butter mixture. Mix just untl combined or a few streaks of flour remain.
Stir in your chosen add-ins.
Spread the batter evenly in the prepared pan. Bake 22 to 25 minutes, or until the edges are browned and a pick inserted into the center comes out clean.
Cool the blondies in the pan on a wire rack. Then use the parchment paper to lift them from the pan onto a wire rack. Cut into bars.
Notes
Store in an airtight container at room temperature up to 3 days.