In a medium bowl combine the flour, cornstarch, salt, baking powder, and baking soda. Set aside.
In a large mixing bowl, beat together the butter, granulated sugar, and ¼ cup of powdered sugar on medium-high until fluffy and light, about 3 minutes. Add the lemon zest, lemon juice, egg yolk and lemon extract. Beat until combined.
Add ⅓ of the reserved flour mixture to the butter and sugar mixture and blend on low until incorporated. Repeat in two more additions, blending just until the flour is incorporated. Scrape down the sides of the bowl and blend for just a few seconds as needed. Scrape down the sides again and gather the dough into a ball in the bottom of the mixing bowl. Cover and refrigerate the dough for at least 1 hour and up to 24 hours.
Preheat oven to 325°F. Prepare 2 baking sheets lined with parchment paper. Roll the dough into 1-inch balls and place two-inches apart on the parchment lined baking sheet.
Lightly flatten each dough ball using the bottom of a small flat drinking glass covered with a piece of wax paper. I use a rubber band to keep the wax paper on the glass.
In a small pie plate or low rimmed bowl, combine the 1 ½ cups of powdered sugar and the lemon crystals. Mix well and set aside.
Bake the cookies for 12 to 14 minutes or until the edges are lightly browned. Remove from the oven and immediately toss a few hot cookies in the powdered sugar mixture, face down, to coat. After a few seconds turn the sugar coated cookies over using a fork. Once completely covered with lemon sugar on both sides, remove with a fork onto a cooling rack and repeat until all hot cookies are well covered. Work quickly to ensure the cookies don’t cool before covering in sugar. Repeat with remaining cookie dough. Don’t let the dough sit on a hot pan for long before baking.
Allow all the cookies to cool for about an hour, then toss again in the powdered sugar. Store in an airtight container.