In a small non stick pot, add the erythritol and eggs. Mix with a whisk until nice and creamy. Add the lemon juice and lemon zest and whisk again until completely mixed in.
Turn on the heat to low-medium and stir constantly for 5-10 minutes until the curd thickens up. Do not put the heat to high as the eggs will scramble. Always cook the curd on low-medium heat.
When the curd has thickened, add the butter and mix in. The heat will make the butter melt and will make the curd nice and smooth.
Transfer the curd to a jar or bowl and cover it with cling film so that the top doesn't dry out. Make sure there is no air pockets and that the cling film is perfectly set on the custard. If not, the custard will crystallize in that area as it cools. Let it cool for an hour or so and then place in the fridge to cool perfectly for 2 hours maximum. Longer and it will tend to crystallize.
When you use it on a dessert, make sure to serve and eat it the same day as the curd will crystallize if you leave it in the fridge uncovered, like on a pie or tart.