Start by making the lemon curd – a day in advance, if possible. Combine the lemon juice, zest, caster sugar and eggs in a heatproof bowl. Place the bowl over a small saucepan of gently simmering water, and heat slowly – whisking continuously. The mixture will slowly turn from a frothy consistency to a thick, velvety consistency – and you will know that it is ready when you can coat the back of a spoon and draw a clear line through it. At this stage, remove the curd from the heat, and whisk through the cold butter until the mixture is rich and glossy. Set aside until ready to use (or place in the fridge overnight).