Boil the closed, sweetened condensed milk in an open pot in a water bath for about 1 1/2 hours. Carefully open the cans and pour the now caramelized condensed milk into a large mixing pot (capacity approx. 2 liters). Now add the vanilla flavor, the gingerbread spice and the rum. Puree with the stirring stick until a nice, even, thick liqueur is created. Now fill into bottles. The consistency is similar to an eggnog.
Makes 2 bottles of 0.7 liters each and a small remainder to try immediately. The correct result can only be tasted after 3-4 days, when the spices have soaked through.