Preheat oven to 350°F. Line an 8-inch square pan with parchment paper and spray lightly with nonstick cooking spray. (I usually spray the pan lightly before I put the parchment paper in, too. It keeps the paper in place.)
In a large bowl, stir together melted butter and brown sugar until smooth. Add egg and vanilla and stir until well combined.
½ cup unsalted butter, melted, 1 cup packed light or dark brown sugar, 1 large egg, lightly beaten, 2 teaspoons pure vanilla extract
Measure out flour, baking powder and salt. Put into bowl right on top of wet ingredients, then stir until just combined.
1 cup all-purpose flour, ½ teaspoon baking powder, ¼ teaspoon kosher salt
Spread batter into prepared pan. Bake for 25 to 28 minutes or until a toothpick inserted in the middle comes out with moist crumbs. The blondies will be slightly browned around the edges. Don't overbake the blondies. They'll continue to cook a bit more even after you take them out of the oven.
Cool for at least 20 minutes in the pan before lifting the blondies out of the pan by grasping the edges of the parchment paper. Set on a wire rack to finish cooling.
Cut into squares and store in an airtight container.
Video
Notes
Add-ins: If desired, add 1 cup butterscotch or white chocolate chips, or ½ cup colored jimmies (sprinkles), or 1 cup chopped toasted walnuts or pecans. Mix into batter after adding dry ingredients or sprinkle on top.
Storage/freezing: Store in an airtight container once the bars have completely cooled. They can be frozen for up to three months.