Combine the milk, cream, vanilla bean (pod and seeds), sugar and cinnamon (stick and ground) in a large, heavy non-corrosive saucepan (preferably enameled cast iron).
Bring to a boil, stirring frequently, then simmer, covered, over medium-low heat for 10 minutes.
Remove from the fire.
Beat the egg yolks until runny.
Slowly whisk in 1 cup of the hot milk mixture, then whisk the yolk mixture into the milk remaining in the saucepan.
Return the pan to medium-low heat and cook, stirring almost constantly, until the custards has thickened enough to coat a wooden spoon (it should be 185-190 degrees), 7 to 10 minutes.
Do not let the mixture come near a boil or the egg yolks will curdle. Immediately strain through a fine-mesh sieve, then stir in the vanilla and cinnamon schnapps and cool to room temperature, stirring occasionally. If there is time, cover and chill before freezing.
Scrape the custard into the canister of your ice cream freezer and freeze according to the manufacturer's directions. For a firmer ice cream, "ripen" for several hours by placing in your freezer.