Prep Preheat the oven to 350°F. Grease an 8×8-inch pan with cooking spray or line it with parchment paper, then set it aside.
Make batter In a large mixing bowl (or stand mixer), cream together the butter, sugar, and lemon zest until the mixture becomes pale and fluffy. Add the eggs one at a time, mixing thoroughly after each addition. Stir in the lemon juice. Add the flour and baking powder, mixing just until everything is incorporated.
Bake Transfer the batter to the prepared pan and spread it evenly. Bake for 20–25 minutes, or until a toothpick inserted in the center comes out clean. Allow the brownies to cool for 10–15 minutes.
Glaze In a small bowl, whisk together the powdered sugar, lemon juice, and lemon zest until smooth. Pour the glaze over the brownies and spread it evenly across the surface.
Cool and slice Let the brownies cool completely before cutting. Slice into 16 squares.
Notes
If using unsalted butter, add a pinch of 1/4 teaspoon salt to the batter.
Store leftovers in a sealed container for up to 3 days. Freeze for up to 3 months.