Preheat your oven to 375℉. Line two baking sheets with silpats or parchment paper.
Fill a deep mixing bowl with warm water and place the unopened can of lemonade concentrate in it. It will thaw (enough) while you make the cookie dough.
In a large mixing bowl, cream together the butter and sugar with a hand mixer. Add the lemon zest and mix again to combine. Now add the eggs one at a time, beating to combine thoroughly between each egg addition. In a separate bowl, whisk together the flour, baking soda, and salt.
Measure out ½ cup lemonade concentrate and set the rest aside. Add about one-third of the flour mixture to the creamed butter and sugar. Beat on low speed until just combined, then add about half the lemonade concentrate. Again, beat until just combined. Continue adding the flour and lemonade alternately, beating on low after each addition, until combined and the mixture shows no streaks of flour.
Use a cookie scoop to drop approximate tablespoonfuls of cookie dough onto the prepared baking sheets, spacing them 2 inches apart. Use your fingers to pat the drops of dough into roughly even balls. You don’t have to be the least bit precious here, but you do want them roughly uniform balls instead of globs with little bits of dough sticking out willy-nilly.
Bake the cookies for 8 minutes. Check them, and if the edges are not starting to brown, give them 1 minute more. The centers will look underdone, but this is okay. Using a pastry brush, immediately brush the tops of each cookie with some of the reserved lemonade concentrate. Then sprinkle lightly with sugar. Transfer cookies to cooling racks and allow them to cool completely.