In a microwave-proof bowl, whisk together the egg yolks, whole egg, sugar, and salt until well combined. Whisk in the lemon juice and lemon zest.
Place the bowl in the microwave and cook in 40-second increments, stirring each time for around 3-4 minutes. This time may increase or decrease depending on your microwave strength. The curd may puff up a little near the end of cooking time. There may be little pieces of cooked egg, but they will be sieved out later.
Check that the curd is thick enough by dipping it in a metal spoon. The curd should thickly and easily coat the back of the spoon.
Stir the cold cubed butter through the hot curd and stir until smooth. Pour the finished curd through a fine-mesh sieve into clean jars. It will thicken more as it cools. Once cooled, store it in the fridge.
Notes
Freezing - Lemon curd can be frozen for up to three months.The cup sizes given are for US cups. Note that these are smaller than metric cup sizes.If you don't have a microwave, you can make this using a double boiler and cook it for around 12-15 minutes until it thickens. After this, stir through the cold butter.