01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt until well combined.
03 - In a large bowl, beat butter and sugar with electric mixer until light and fluffy, approximately 2-3 minutes.
04 - Beat in eggs one at a time until incorporated. Add lemon juice, lemon zest, and vanilla extract, mixing until combined.
05 - Gradually add dry ingredients to wet mixture, mixing on low speed until just incorporated. Do not overmix.
06 - Drop rounded tablespoons of dough onto prepared baking sheets, spacing 2 inches apart to allow for spreading.
07 - Bake for 10-12 minutes, or until edges are lightly golden and centers are set.
08 - Allow cookies to rest on baking sheets for 5 minutes, then transfer to wire racks to cool completely.
09 - Whisk together powdered sugar, lemon juice, lemon zest, and melted butter if using, until smooth and spreadable.
10 - Once cookies are completely cool, spread frosting over tops. Let set for 15 minutes before serving.