For dark chocolate layer, in a small heavy saucepan combine 1 cup of the cream, the dark chocolate, and 3 tablespoons of the sugar. Cook and stir over medium heat about 5 minutes or until mixture comes to a full boil and thickens. Remove from heat. In a medium bowl lightly beat 2 of the egg yolks with a fork. Gradually stir about half of the hot mixture into egg yolks. Return egg yolk mixture to saucepan. Cook and stir for 2 minutes. Stir in 1/2 teaspoon of the vanilla; cool slightly.
Divide dark chocolate mixture evenly among eight 2- to 3-ounce glasses, dishes, pot de crème cups, or ramekins. Cover and chill for 2 hours or until firm.
For white chocolate layer, in another small heavy saucepan repeat Step 1 using remaining cream, the white chocolate, and remaining sugar, egg yolks, and vanilla. Divide white chocolate mixture evenly among chilled glasses. Cover and chill about 4 hours or until firm. Garnish with white chocolate shavings, if desired.
If desired, substitute milk chocolate for the white chocolate layer or milk chocolate for the dark chocolate layer.