Make The Cookie Dough
In the bowl of a stand mixer fitted with a paddle attachment, cream the butter, both sugars, lemon zest and salt on high speed until light and fluffy, about 3 minutes.
Add in the egg, lemon juice and lemon extract and beat until combined, scraping down the sides of the bowl once or twice.
In a medium bowl, whisk together the flour, cornstarch and baking soda, then turn the mixer to low speed and gradually stir in the flour mixture until evenly combined.
Divide the dough into 12 equally sized mounds (about ¼ cup/60 ml) and roll into balls. Place the dough on a parchment-lined baking sheet, cover with plastic wrap and refrigerate until firm, at least 3 hours and up to 3 days.
When ready to bake, preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper.
Place the dough mounds 2 inches (5 cm) apart on the prepared sheet and bake for about 10 minutes, until the tops and edges are just set (don’t overbake).
Let cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.
Make The Lemon Glaze
In a small bowl, combine the powdered sugar and 2 tablespoons lemon juice until smooth. If a thinner glaze is desired, add the extra tablespoon of lemon juice.
Dip the top of each cookie in the glaze, then return to the wire rack until the glaze is set.
Enjoy with a cup of tea. Store in an airtight container for up to 2 days.