01 - Preheat oven to 350°F. Line 8x8-inch baking pan with parchment paper.
02 - Whisk together flour, baking powder, and salt in medium bowl. Set aside.
03 - Whisk melted butter and sugar until smooth. Add eggs one at time, whisking after each. Stir in lemon juice, zest, and vanilla.
04 - Gently fold dry ingredients into wet mixture, mixing just until combined. Avoid overmixing.
05 - Pour batter into prepared pan, smooth top. Bake 22-25 minutes until center set and edges lightly golden. Cool completely in pan on wire rack.
06 - Whisk powdered sugar and lemon juice until smooth. Spread over cooled brownies and sprinkle with extra zest if desired.
07 - Cut into 16 squares and serve.