Preheat the oven to 325°F (160°C) and line a baking sheet with parchment paper.
In a mixing bowl, cream together the softened Irish butter, powdered sugar, and granulated sugar until light and fluffy.
Add the vanilla extract (and citrus zest if using) to the butter mixture and mix until combined.
In a separate bowl, whisk together the all-purpose flour, cornstarch, and salt.
Gradually add the dry ingredients to the butter mixture, mixing gently until the dough comes together. It should be soft but not sticky.
Turn the dough onto a lightly floured surface and roll it out to about ¼ inch thickness.
Use a cookie cutter to cut out shapes and place them on the prepared baking sheet. Alternatively, you can shape them into a log, chill, and slice for a different presentation.
Bake in the preheated oven for 15-18 minutes, or until the edges are lightly golden.
Remove from the oven and let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
Notes
Arrange the cookies on a decorative plate and dust with a little extra powdered sugar before serving for an elegant touch.