Place egg yolks, sugar and salt into a large non-stick pan and whisk until well combined and smooth.
Place milk and double cream into a large jug and briefly whisk together.
Place pan with egg yolks and sugar over a low-medium heat and attach a candy thermometer. Add a splash of the milk/cream mixture and whisk until combined.
Continue very gradually adding milk and whisking between each addition to ensure there are no lumps. Switch from a whisk to a wooden spoon once all the milk is added.
Keep stirring and cooking gently until the candy thermometer reaches 160F/70C – this is the temperature at which the eggs are safe to consume.
At this stage the mixture will begin to thicken; keep cooking until the mixture coats the back of a spoon. My mixture got to 180F/82C and was then thick enough to coat my spoon. It won’t be super thick, but you will feel slight “resistance” when stirring.
Make sure you don’t boil the mixture or put it on too high a heat or you will end up with scrambled eggs!
Remove from the heat and add the vanilla extract, stirring to combine.
Leave to cool for 15 minutes before placing a layer of cling film over the top of the eggnog; touching the top of the eggnog so that no skin develops as the eggnog cools.
Once completely cool, place in an airtight container and store in the fridge. Since this is alcohol free eggnog, it won’t last as long as the alcoholic version; so should be consumed within 3 days.