Score the vanilla pod and, with the back of a knife, scoop out all the inner seeds.
Place the milk, grated lemon zest and vanilla stick seeds in a small saucepan and stir.
Let the milk heat up.
Whisk the egg yolks together with the sugar for a few minutes to make a frothy, whitish mixture then add the cornstarch and continue whisking a few seconds.
When the milk is about to boil put the beaten egg mixture in the center of the saucepan and do not stir.
Cook for a few minutes until you will see small volcanoes forming on the surface.
Only at this point stir very quickly with a hand whisk and cook for about 5 minutes.
Let the custard cool completely and add it to the limoncello.
Mix and blend the limoncello with the rest of the custard.
The limoncello custard is ready; you can use it as you wish.
