• Cream the soft butter, lemon zest, and powdered sugar well, for at least 2-3 minutes, scraping down the bowl a couple of times. This aerates the dough for a lighter cookie.
• Blend in the lemon juice and egg yolk.
• Turn the mixer to low and blend in the flour and cornstarch until the dough comes together and there is no dry flour left. It will be soft and sticky.
• Form the soft dough into a flat disk and wrap in plastic wrap, smoothing out the surface as you wrap it.
• Put the dough in the refrigerator for 2 hours or in the freezer for 30 minutes to firm up.
• Preheat oven the 350F and line baking sheets with parchment paper.
• Roll out the dough to 1/4 in thickness on a lightly floured surface. If your dough is super cold it will soften as it sits at room temperature.
• Cut into rounds with a small 2 inch cookie cutter and place on a parchment lined baking sheets, one inch apart. If your dough has warmed up you may want to chill the cookies for 15-30 minutes before baking. I do a test cookie first to evaluate. If it doesn't spread, you're good to go.
• Bake for 8-10 minutes until set. They will still be pale.
• Let cool before glazing.

to make the glaze

• Stir together the sugar and lemon juice until smooth and glossy. I don't find it necessary to use sifted sugar: if you have a few lumps in your glaze let it sit for a few minutes and stir again. Any small lumps should dissolve on their own.
• Take each cooled cookie and dip headfirst straight down into the glaze, just touching all parts of the surface to the glaze. Lift the cookie straight up and let the excess glaze drip off. Turn the cookie upright and set on a rack. Dust with more lemon zest while the glaze is still wet.
• Allow the glaze to set up before eating or storing your lemon cookies.
• Cookies will remain fresh for several days at room temperature, and can be frozen for up to 3 months.