The base for these cookies is a Breton-style of butter cookie, that is tender and delicate and has a noted little salt kick to them. Because they are so tender, decorating with a royal icing makes them stronger but also adds sweetness, since the cookies themselves aren’t overly sweet.
If you have access to good quality butter (82-84% butterfat) that is ideal, but the cookies will still turn out if you don’t.
For the cookies, beat the butter and icing sugar until smooth and light using electric beaters or in a stand mixer fitted with the paddle attachment, on medium speed. Add the egg yolk and lime zest and beat in. Add the flour, cornstarch and salt and mix in on low speed until the dough comes together (the dough will be very soft.) Wrap the dough in plastic and chill for at least 2 hours before rolling.
Preheat the oven to 325 ºF (160 ºC) and line 2 baking trays with parchment paper.
On a lightly floured work surface, knead the dough slightly to soften (this prevents cracks when rolling the dough.) Roll the dough out until it is ¼-inch (6 mm) thick and use a cookie cutter of any desired shape to cut out the cookies (a 2 ¼-inch/6 cm round cutter yields about 30 cookies) and place them onto the trays. Bake the cookies for 8-10 minutes, until they barely start to brown at the edges. Allow the cookies to cool on the trays before icing them.
For the royal icing, whip the meringue powder and water (or 1 egg white) with the icing sugar in a bowl using an electric beaters, or a stand mixer fitted with the whip attachment, until peaks form (approximately 8-10 minutes). This will make an icing that is the ideal consistency for piping precise lines. For a “flood” style of royal icing, add additions of water, a few droplets at a time, until the icing becomes more fluid and flows without being runny. If you wish to add colour to the icing, divide the icing between a few bowls and tint as desired. If not using the icing immediately, cover the bowls so that the plastic wrap sits directly on the icing to prevent it from drying.