This is rich, but if you are hungry for a true vanilla mousse, this is it! A bit of work, but worth the effort.
1 1/2 cups granulated sugar
1 cup water
8 egg yolks
1 vanilla bean, split lengthwise
1 quart heavy cream
Pure vanilla extract to taste
Boil sugar and water rapidly for 5 minutes. Cool. Beat egg yolks on top of a double boiler and whip into the syrup gradually. Add vanilla bean, and cook custard over very hot, but not boiling water, stirring constantly, until it becomes creamy and thick. Remove vanilla bean, and squeeze, pressing out seeds. Rub custard through a sieve and stir over a bowl of ice water until it cools.
Whip 1-quart heavy cream until it is stiff enough to hold a shape, add vanilla extract to taste, and fold the cream into the cooled custard.
This can be frozen and served half-frozen, or it can be spooned into cups and served chilled. Decorate with crystallized flowers, shaved chocolate, etc. if desired. Flavored liqueurs can be added instead of vanilla extract for a delicate secondary-flavor, if desired.
Serves 6 to 10, depending on serving size.