You can make this curd using two different methods: you can cook it indirectly in a double boiler or over direct heat in a sauce pan. The indirect method almost guarantees that you will not over-cook it, while the direct method requires careful attention and it's best to sieve the finished curd in this case.

• Combine sugar and lemon zest in a heatproof bowl for the indirect method or in a small saucepan for the direct method. Use the back of a spoon or your fingertips to rub the zest into the sugar. This will coat the sugar crystals with fragrant oils from the lemon peel, adding a whole new dimension of intense lemon flavor to this curd.
• Add the whole egg and extra egg yolk (if you're making the rich & thick version) to the lemon sugar and whisk until smooth. Whisk in lemon juice. Add the pieces of butter and set the bowl over a pot with ½-inch of simmering (or use a double boiler). Whisk gently over the for 7-10 minutes until thick enough to coat the back of a spoon (using a spatula prevents the incorporation of many tiny air bubbles that whisking would otherwise cause). If it still hasn't thickened after 10 minutes, increase the heat.
• If you're cooking over direct heat, then place the saucepan over low heat and whisk gently but constantly until the mixture thickens. Do NOT walk away or stop stirring. If the mixture heats too fast, then it can curdle.
• Immediately pour the hot curd through a sieve and into a small jar or glass storage container, place a piece of plastic wrap directly in contact with the surface and refrigerate until thoroughly chilled. Once chilled, cover it tightly and keep refrigerated. This curd will last for about a week in the refrigerator.