Beat the egg yolks and sugar in a china or glass bowl until they become pale yellow and creamy: add the juice and grated rind of the ½ lemon, combining them with the egg, then add the flour a little at a time, mixing it in well, lastly, gradually stir in the milk. Pour this “cream” into a small stainless steel saucepan (or milk pan) and put it over a very low heat, stirring continuously until it becomes nice and thick. Take it off the stove and stir in the Vin Santo. Put the Cantucci in a plastic bag: hold it closed with one hand and, with the other, hit the biscuits with a rolling pin until they are coarsely broken up. Add them to the cream (leaving 3-4 spoonfuls to one side) and stir carefully.
You can serve it in a bowl (with the remaining “crumbs” of Cantucci sprinkled on top) or, for a fun idea, fill small glasses with the cream and decorate each one with the crumbled biscuits. Before “offering” your glasses, leave them for 10-15 minutes in the freezer to make the cream more compact.
You can also make a similar cream with coffee, nice and strong, instead of the Vin Santo, or with half of one and half of the other: they’ll be delicious whichever way you choose.