Put the milk, vanilla and lemon zest in a pot and mix it eventually. It is important that the fire is in the middle level of the kitchen.
At the same time, have a medium cup. Place egg yolks and sugar, beat until smooth. Add the cornstarch, mix constantly to avoid lumps.
When the milk begins to boil, add little by little the egg mixture and the flour that we have prepared in the previous step.
To finish making the pastry cream, mix constantly for 7 minutes, you will notice how the cream begins to thicken. Remove from heat and allow to cool to room temperature.