Crema pasticcera is best eaten within two or three days (if kept in the refrigerator) and does NOT freeze well.
This recipe makes enough cream to fill one 9 inch pan di spagna.
Heat the milk on the stove (but don't boil it). In a large bowl, mix the yolks, the sugar, the flavors (which are optional) until pale and thick. You can add the alcohol now if you want to make it evaporate a little bit. Then slowly add the flour (sifting it on the batter) and keep mixing until smooth and well blended. Pour the warm (but not boiling) milk on the batter, a little at a time, and mix until blended.
If you are using a microwave, put the bowl in the microwave, and cook it on high for 2 minutes. Take the bowl out and stir the content with a woodden spoon. Then cook it again (on medium) for additional increments of 30 seconds or 1 minute minutes. Each time take the bowl out and stir vigorously. The time it takes to cook in the microwave ovens varies a lot across ovens, so you should experiment. In any case, do not overcook, otherwise the cream will taste of flour. Be careful in particular if you have a very recent, powerful microwave oven. In this case, go slow, above all towards the end.
If you are cooking it on the stove, pour the batter in a deep pan over low fire, and keep stirring (slowly) until the cream thickens. You must stir the mixture to avoid burning (the bottom will burn if not stirred), but you don't want to stir too much, because the more you stir, the less the cream will thicken. It should take from 5 to 10 minutes, but again the time varies. Remove from the fire and pour it in another container. As before, do not overcook.
You can add the alcohol at the end, when the cream is cooling, if you don't want the alcohol to evaporate.
If the cream seems to have too many lumps, try passing it through a sifter; although you're probably better off making a new batch.
Note that crema pasticcera can be made with potato starch or corn starch instead of flour, which makes it edible for people allergic to gluten. I don't know the proportions for these starches though, my guess would be a little less than the dose you would use with flour, but I must try.