In a bowl, rub the lemon zest into the sugar to release its natural oils. Then whisk in the melted butter
Add the egg, lemon juice, and vanilla. Mix until smooth
Sift in the flour, cornstarch, baking powder, and salt. Fold gently until just combined—don't overmix
Scoop the dough using a large ice cream scoop and refrigerate for at least 15 minutes (or up to overnight)
Preheat the oven to 350°F (180°C)
Roll the chilled dough between your hands, then gently press it onto a lined baking sheet
Bake for 9-11 minutes—don't overbake! The cookies should be soft with a slightly gooey center
Let them cool on the pan for 10 minutes before transferring to a wire rack to cool completely
In a bowl, cream all the frosting ingredients together using a spatula or hand mixer until smooth
Transfer to a piping bag with a 2A tip (or simply snip the end)
Frost the cookies and sprinkle with extra lemon zest (optional, but highly recommended)
