Thoroughly mix milk, condensed milk, cinnamon stick and egg yolks. The egg yolks should be homogeneously integrated into the liquid.
Then heat in a bain-marie for about 25 minutes while stirring. Add vanilla sugar and make sure the liquid does not get hotter than 80 degrees. I used roasting thermometers. Otherwise the ice cream tastes too much like egg.
Then let the liquid cool. Remove the cinnamon stick. Whip cream (not too stiff) and fold in. Put in the freezer, take out after an hour and stir through. Once the ice cream is semi-frozen and creamy, incorporate the coarsely chopped cookies into the mixture. Stir the ice cream every hour.