Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Whisk melted butter, sugar, and lemon zest in a bowl until the mixture looks creamy and fragrant.
Whisk in the eggs until smooth, then stir in vanilla and lemon juice.
Combine flour, baking powder, and salt. Fold into the wet mixture just until no dry streaks remain.
Spread batter in the pan and smooth the top. Bake 20–24 minutes, until edges are set and center barely springs back.
Cool in the pan for 10–15 minutes while you make the glaze.
Whisk powdered sugar with lemon juice, optional zest, and melted butter until silky and pourable.
Pour glaze over warm brownies and spread to the corners. Let set until the glaze firms.
Lift out with parchment, slice into squares, and serve.