Bring the milk to the boil with the split vanilla bean. Do not boil further. If the bean is not to be used, add the vanilla extract later. Remove the vanilla bean, if used, rinse it off, dry it, and reserve for later uses.
Place the yolks in a saucepan and add the sugar. Beat with a wire whisk until thick and lemon-colored.
Gradually add the milk to the yolk mixture, beating constantly. Use a wooden spoon and stir constantly, this way and that, making certain the spoon touches all over the bottom of the saucepan. Cook until the mixture has a custardlike consistency and coats the sides of the spoon. Do not let the sauce boil or it will curdle. Add the vanilla extract if used.
Soften the gelatin in the water and add it to the sauce, stirring to dissolve. Add the liqueur and strain into a bowl. Let cool.
Whip the 2cups heavy cream until stiff and fold it into the sauce.
Rinse out a 10- to 12-cup ring mold without drying. Sprinkle the insides with sugar and shake out excess. Add the custard mixture and place in the refrigerator. Chill several hours or overnight until the custard is set.
When ready to serve, dip the mold into hot water and remove immediately. Wipe off and unmold. A damp, hot cloth could also be used on the mold to help loosen the Bavarian cream. Serve with English custard, and, if desired, whipped cream.