Preheat the oven to 150C/130C Fan/Gas Mark 2. Line 2 large baking trays with greaseproof paper (or silicone liners).
In a large bowl, beat the butter for several minutes to soften – best done with a stand/electric mixer. Add the sugar and continue to beat until fully incorporated. Add the flour, lime zest and salt and beat again until it comes together. You’ll know it’s done when you it will hold together when you press it with your hands – a solid dough.
Roll out onto a lightly floured surface to about 1-2cm thick. Cut about 15 rounds with a small circular cookie cutter and place on the lined trays – they don’t need to be too spaced out as they won’t spread very much.
Bake for 15-20 minutes, until only just turning golden at the edges. They will continue to cook once removed. Remove and leave to cool.
Once the cookies are cool, make the glaze by combining the icing sugar and lime juice. It should be very thick, only just spreadable.
Spread about 1/2 tsp of the glaze over each cookie, grate over more lime zest and let set (about 20 mins) before eating!