Put the eggs and sugar in your saucepan and whisk to combine well.
Whisk in the butter cubes, zest and juice.
Place on the stove on gentle heat and gradually bring to a low simmer. Stir all the time with the whisk as it will thicken quickly.
When it is the consistency of thickened or dollop cream, run it through your fine sieve into a small bowl. This step is necessary if you are needing it to be completely lump free, and remove the zest. But omit the sieving if that is not required.
If you are not making it into a mousse straight away, divide into two sterilised jars and seal. It will keep in the fridge for up to 2 weeks.