[Thanks to Sara, author of the Flatbush Food Coop post below, for this recipe.]Brewhaus.com. I am guessing you could also substitute another vanilla-flavored liqueur.
Make yourself a batch; consider it inspiration for the Ice Cream Contest (deadline: August 14)
2/3 cup sugar
2/3 cup water
1 cup molasses
1 cup root beer
1/4 cup Liqueur 45 (see note, above)
1 tablespoon lemon juice
In a small heavy saucepan, combine sugar and water and bring to a boil, stirring until sugar is dissolved. Remove pan from heat.
In a blender, combine molasses, root beer, Liqueur 45, and sugar syrup until smooth. Force purée (push through, a wooden spoon helps) through a fine sieve set over a bowl, and stir in lemon juice.
Chill mixture until cold and freeze in an ice-cream maker according to manufacturer's instructions.