It's like a magic trick to pull a can of lemonade out of the freezer and say to the kids, "Do you think we can make this lemonade into cookies?" If you want to make the cookie dough ahead of time, it can be refrigerated for several days if you seal it in two layers of plastic wrap. Dough straight from the refrigerator should bake for about 12 minutes; dough at room temperature bakes in 8-10 minutes.
1. Preheat the oven to 375 F.
2. Fill a large bowl with warm water and plop in the unopened can of lemonade to thaw while you mix the cookie dough.
3. In a large bowl, cream together the butter and sugar with a hand mixer. Add the lemon zest, if desired. Add the eggs one at a time, beating well after each addition. Continue beating until the mixture is light and creamy, 4-5 minutes, then set aside. In a separate bowl, whisk together the flour, baking soda and salt.
4. Measure out 1/2 cup of the thawed lemonade and set the rest aside. Add about 1/3 of the flour mixture to the creamed butter and sugar. Stir, then add about one third of the 1/2 cup lemonade. Continue adding the flour and lemonade alternately, stirring after each addition, until the mixture shows no streaks of flour. Beat on low speed with the mixer just until all the ingredients are combined.
5. Drop rounded teaspoonfuls of the dough 2 inches apart on an ungreased baking sheet.
6. Bake the cookies, checking them after 8 minutes. When the edges are just starting to brown, remove the cookies from the oven. (The centers will still look soft.) Using a pastry brush, immediately brush the top of each cookie very lightly with a little of the remaining lemonade concentrate, then lightly sprinkle with sugar. Transfer the cookies to a rack and allow them to cool completely.