01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
03 - In a large bowl, whisk melted butter and sugar until smooth. Add eggs one at a time, whisking after each addition. Stir in lemon juice, lemon zest, and vanilla extract.
04 - Gently fold in the dry ingredients, mixing just until combined. Do not overmix.
05 - Pour the batter into the prepared pan and smooth the top. Bake for 22-25 minutes, or until the center is just set and the edges are lightly golden. Allow to cool completely in the pan on a wire rack.
06 - For the glaze, whisk powdered sugar and lemon juice until smooth. Spread over cooled brownies and sprinkle with extra zest if desired. Cut into 16 squares and serve.