01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring it overhangs slightly for easy removal.
02 - Whisk together flour, baking powder, and salt in a medium bowl until well blended. Set aside.
03 - In a large bowl, whisk melted butter and sugar until smooth and creamy. Add eggs one at a time, beating thoroughly after each addition. Stir in lemon juice, lemon zest, and vanilla extract.
04 - Gently fold the dry mixture into the wet ingredients using a spatula. Mix just until combined, being careful not to overmix.
05 - Pour batter into prepared pan and smooth the top evenly. Bake for 22-25 minutes until center is just set and edges are lightly golden.
06 - Allow brownies to cool completely in the pan on a wire rack before glazing.
07 - Whisk powdered sugar and lemon juice in a small bowl until smooth. Spread evenly over cooled brownies and sprinkle with additional lemon zest if desired.
08 - Cut into 16 equal squares and serve. Store in an airtight container at room temperature for up to 3 days.