Preheat oven to 350 degrees F. Prepare a 9x9-inch baking pan by spraying it with nonstick spray. Line with parchment paper for easy removal. Set aside.
In the bowl of your stand mixer fitted with the paddle attachment or in a large bowl with an electric mixer, add butter, sugar, and lemon zest. Beat until light and fluffy, about 3 minutes on medium, scraping down the bowl as needed.
Add flour, baking powder, and salt. Stir in until just combined.
2 ½ cups all purpose flour, 1 teaspoon baking powder, ¼ teaspoon salt
Pour the batter into the prepared baking pan. Use a spatula to spread into an even layer.
Bake for about 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let brownies cool in the pan.
Once cool, make the lemon glaze. In a small bowl, whisk together the powdered sugar and fresh lemon juice.
1 cup powdered sugar, 1 ½ tablespoons fresh squeezed lemon juice
Pour on top of the brownies.
Let the glaze set up and harden for a few minutes before slicing and serving.
Notes
Butter: If using unsalted butter, increase the salt to 1/2 teaspoon.
Lemon: It's important to use fresh lemons. The lemon zest and lemon juice are what give the brownies their bright flavor.
Flour: To properly measure the flour, either weigh it (click "metric" just above the ingredients for the weights) or sift, lightly spoon into a measuring cup, and level. Avoid packing the flour as that will give you dense, dry brownies.