A lipsmacking dessert which can make any party's menu list A class

2 cups chilled heavy cream
4 large egg yolks
3 tablespoons sugar
1 teaspoon vanilla
¹/² cup bittersweet chocolate (not unsweetened),chopped
Garnish: lightly sweetened whipped cream
                 Cake crumbs
                  Oreo biscuits crushed

1.Heat 3/4 cup cream in a  heavy saucepan until hot.
2. Whisk together yolks, sugar, and a pinch of salt in a metal bowl until combined well, then add hot cream in a slow stream, whisking until combined.
3.Transfer mixture to saucepan and cook over moderately low heat, stirring constantly.
4.Pour custard through a fine-mesh sieve into a bowl and stir in vanilla.
5.Melt chocolate in a double boiler or a metal bowl set over a pan of simmering water, stirring frequently. Whisk custard into chocolate until smooth, then cool.
6.Beat remaining 1 1/4 cups cream in a bowl with an electric mixer until it just holds stiff peaks. Whisk one fourth of cream into chocolate custard to lighten, then fold in remaining cream gently but thoroughly.
7.Take a glass.Put a layer of Oreo biscuits and then cake crumbs.Pour 2 spoon of mousse and repeat it,and chill, covered, at least 6 hours.
8.Let stand at room temperature about 20 minutes before serving.