Cream the butter and sugar together until pale and fluffy.
Grate in the lime zest and cream some more.
Sift in the flour and baking powder and stir together - the mixture is a little stiff, but persevere.
Squeeze in the juice of half a lime (about 1 tbsp) and stir.
Mix in the chocolate, if using.
Bring the mixture together with your hands and roll out on a floured surface to about 1 cm in thickness.
Cut out circles or heart shapes. Re-roll the trimmings and repeat the process, creating about 30 biscuits in total.
Bake on a lined baking sheet in the middle of the oven, one batch at a time, at 180℃ (350℉, Gas 4) for 10 to 15 minutes. The biscuits should be golden and crisp around the edges, but not too brown
Leave to firm up for a couple of minutes then transfer to a wire rack. Dust the tops with caster sugar and leave to cool.
These biscuits have a habit of spreading, so it's a good idea to leave them in a cool place to firm up for thirty minutes or so before you pop them into the oven.Please note: calories and other nutritional information are per serving. They're approximate and will depend on serving size and exact ingredients used.