Preheat oven to 350°F. Line a baking sheet with parchment paper. Set aside.
In a large bowl, using a hand mixer, beat the butter, powdered sugar, and lemon zest until combined.
1 cup unsalted butter, ⅔ cup powdered sugar, 1 tablespoon lemon zest
Add the flour and salt and mix by hand until the dough starts to clump. Gently press the dough into a ball.
2 cups all-purpose flour, ¼ teaspoon kosher salt
Place the dough on the prepared baking sheet and gently pat into a circle or rectangle.
Bake for 22-25 minutes until just turning light golden at the edges.
As soon as the shortbread is out of the oven, cut into 16 rectangle fingers with a sharp knife, and prick each finger with a fork.
Leave for 5-10 minutes before carefully transferring to a wire rack to cool completely.
While the shortbread cools, make the icing. Sift the powdered sugar and mix with the lemon juice and water.
1 cup powdered sugar, 1½ tablespoons lemon juice, 1 tablespoon water
Spoon ½ teaspoon of icing to the end of each cooled shortbread finger.
Sprinkle over lemon zest, if using.
Lemon zest