Heat the oven to 350°F (175°C). Grease an 8- or 9-inch square baking pan and line it with parchment paper, leaving an overhang on two sides so the cooled brownies can be lifted out.

Whisk the melted, slightly cooled butter and granulated sugar together until smooth. For especially fragrant brownies, rub the lemon zest into the sugar before combining it with the butter.

Whisk in the eggs, lemon zest, and fresh lemon juice until the mixture looks smooth and evenly combined. Room-temperature eggs blend more easily, although the recipe still works if your eggs are cool.

In a separate bowl, whisk together the all-purpose flour, baking powder, and fine salt. This distributes the baking powder and salt before they are added to the wet ingredients.

Fold the dry ingredients into the lemon mixture just until no dry flour remains. Do not overmix, because excess stirring can make the brownies less tender.

Spread the batter evenly in the prepared pan. Smooth the top so the brownies bake to a consistent thickness.

Bake for approximately 22–30 minutes, until the edges are set and lightly golden and the center is just set. A toothpick inserted near the center should come out with a few moist crumbs, not wet batter.

Cool the brownies completely in the pan before adding the glaze. They firm up as they cool, so do not wait for the center to look completely dry in the oven.

Whisk the powdered sugar with enough fresh lemon juice to make a thick but pourable icing, adding the optional teaspoon of lemon zest if desired. Spread or drizzle the glaze over the cooled brownies and let it set before slicing.