Begin by making the lime curd. Place the lime zest and juice, sugar and butter into a large heatproof bowl over a pan of barely simmering water, making sure the base of the bowl is not in contact with the water, and stir every so often until the butter has completely melted. Add the eggs and whisk until combined. Continuing on a low heat, leave to cook for 8-10 minutes, stirring occasionally, until the mixture is thick enough to coat the back of a spoon. Remove from the heat, and set aside to completely cool.