Cut the butter into smallish pieces (1cm or so, but no need to get too fussy) and set aside.
Place the lemon juice and caster sugar into a large, microwave-safe jug. Microwave at 100% power for 2 to 3 minutes, stirring after each minute, until the sugar has dissolved.
Add the butter, and stir with a whisk until the butter is completely melted.
Whisk in the eggs and half (2 teaspoons) of the grated lemon zest.
Microwave for 2 to 3 minutes on 50% power, stirring after every minute, until the curd begins to thicken.
Then drop the power to 30% and cook in 30-second bursts until thick. If your microwave doesn't go to 30% power, you can keep using 50% but make sure you stop and stir it often.
While the curd is cooking, place the other half of the grated lemon zest (2 teaspoons) into a large heatproof jug or bowl, and set a sieve on top of the bowl.
Once the curd is cooked, pour it immediately through the sieve. Give it a stir to incorporate the zest.
Cool for 20 minutes or so, stirring occasionally, then place some plastic wrap directly onto the surface of the curd, and place in the refrigerator to cool completely. Once cold, you can use your lemon curd in your chosen recipe, or transfer it to jars or containers.
Microwave lemon curd will keep for 1 - 2 weeks in the fridge.
Notes
Lemons - Depending on the kind of lemons you use, you may need to adjust the sugar and lemon juice amounts to get the amount of acidity that you like. Once the curd is cooked, give it a taste. If it's too sweet you can whisk in a little more lemon juice, if it's too tart, then whisk in a little icing sugar.Lemon Zest - I don't like to have toooo much lemon zest in my finished curd, so I add half of the lemon zest at the start of cooking to get the flavour, then strain the curd and add the rest of the zest. If you want to keep all of the zest in the curd, just add all of it right at the end.Amount - To make more or less lemon curd, use the adjustable buttons above. Keep in mind if you're making less curd, it will take less time to cook, and if you're making more, it will take longer.Cake filling - If you want to use this as a cake filling, you will need to pipe a dam of firm icing such as buttercream or ganache around the edge of the layers of cake before filling the middle with the curd.