Cream the butter and sugar together with the zest. Gradually add the flour and cornstarch and mix until the dough comes together.

Finish bringing the dough together with a big spoon until there are no dry crumbly bits. Cover with plastic wrap and refrigerate for an hour.

Preheat the oven to 350F

Using a small scoop, or a tablespoon measuring spoon, scoop out dough and roll into a ball. Place on an un-greased baking sheet 2 inches apart and bake for about 11-12 minutes. They will still be pale, but may look slightly golden around the bottom edge. (Longer cooking will result in a crunchier cookie, shorter cooking time results in a softer shortbread.) Let cool on baking sheet for a few minutes before transferring to a rack to cool completely before frosting.

To make the frosting, blend the soft butter with the sugar, adding enough juice to get it to a spreadable consistency. If it becomes too thin, add a little more sugar. Stir until it is creamy, with no sugar lumps. Frost each cookie and garnish with a sprinkle of zest.