In a bowl, beat the softened butter with lemon zest and powdered sugar thoroughly for 2 to 3 minutes, scraping the sides to lighten the mix.
Add lemon juice and the egg yolk, mixing until fully incorporated.
On low speed, gradually add the flour and cornstarch until a sticky dough forms without any dry bits remaining.
Shape the dough into a flat disk, wrap it tightly in plastic, smoothing the top, then chill in the fridge for 2 hours or freeze for 30 minutes to firm up.
Heat the oven to 350°F and prepare baking sheets with parchment paper.
Roll out the chilled dough to about 1/4 inch thickness on a floured surface; if too cold, let it soften a bit at room temperature.
Use a 2 inch cookie cutter to cut rounds and arrange them about 1 inch apart on the parchment-lined baking sheets; if the dough softens, chill cut cookies 15-30 minutes before baking.
Bake the cookies for 8 to 10 minutes until they are set but still pale.
Allow cookies to cool completely before applying the glaze.
For the glaze, stir powdered sugar and lemon juice together until smooth and shiny; wait and stir again if lumps remain.
Dip each cooled cookie quickly headfirst into the glaze, ensuring full coverage, then lift and let excess drip off.
Set the glazed cookies upright on a rack and sprinkle additional lemon zest over the wet glaze.
Let the glaze harden before serving or storing.
Store the cookies at room temperature for several days or freeze for up to 3 months.
