In a medium mixing bowl, add room temperature butter, granulated sugar, lemon zest, salt, egg yolk, and fresh lemon juice. Beat together until light and fluffy.
In another medium mixing bowl, add flour, arrowroot, and baking powder. Mix these with a mixing spoon until well blended. Add this mixture to the first mixture. Blend well with mixer or mixing spoon. Then beat for 2 minutes.
Cover the dough with an airtight wrap and refrigerate for at least an hour. These can be mixed ahead and baked the next day.
Preheat oven to 350F (180C) Remove dough from refrigerator and scoop dough to make small balls and place on parchment-lined (optional) cookie sheet. Flatten each one with a sugared glass bottom or the palm of your hand.
Bake in preheated oven 9-10 minutes. Remove from oven and allow to cool to safely handle to roll in confectioner's sugar until nicely coated. I like the sugar thick enough that you don't see the cookie! This may take a second roll in the sugar. The first sugar may melt; roll again until you have the desired effect.
Notes
Use unsalted butter. Substitutes will ruin the classic flavor. A small cookie scoop is quick and assures the cookies are consistent in size and will bake evenly. Beating the first ingredients and using arrowroot makes a soft, tender cookie with a crisp edge.Alternatively, instead of using a cookie scoop, the dough can be rolled into a log, then sliced discs.Using limes or a combination of lemons and limes (use one of each) works well, too.