Preheat the oven at 180°C (160°C fan, gas mark 4).
Mix the egg whites, caster sugar and desiccated coconut together with a fork to make a soft paste. Stir in the lime zest if you choose to use this ingredient.
Shape into 8 small mounds on a parchment lined baking tray and bake for 12 minutes until just very lightly browned.
Melt the dark chocolate and once the macaroons are fully colled dip the bases into the chocolate. Allow to set on greaseproof paper. Drizzle with the remaining chocolate over the top of the macaroons. These cakes will keep for 1 week in a cake tin.