First you need to prepare the cream because it needs time to cool down.
In a saucepan, combine the egg, salt, vanilla, and sugar. Mix and add the milk. Put on the stove and cook until light thickening on the fire below average. It took me about 10 minutes. In this cream, the egg-milk mass will be quite liquid, not the same as for custard. Remove from the heat and add the lemon zest and juice. Stir. Cool.
And when the egg-milk mass has cooled, I beat the softened butter until it is fluffy and slightly white.
And then, in a few passes, I connect the two masses.
Whisk until smooth and the cream is ready!!!