2 cups chilled heavy cream
4 large egg yolks
3 tablespoons sugar
1 teaspoon vanilla
7 oz fine-quality bittersweet chocolate (not unsweetened), chopped
Garnish: lightly sweetened whipped cream
Special equipment: an instant-read thermometer
Heat 3/4 cup cream in a 1-quart heavy saucepan until hot.
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. Transfer mixture to saucepan and cook over moderately low heat, stirring constantly, until it registers 160°F on thermometer. Pour custard through a fine-mesh sieve into a bowl and stir in vanilla.
Melt chocolate in a double boiler or a metal bowl set over a pan of simmering water (or in a glass bowl in a microwave at 50 percent power 3 to 5 minutes), stirring frequently.
Whisk custard into chocolate until smooth, then cool.
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.
Spoon mousse into 8 (6-ounce) stemmed glasses or ramekins and chill, covered, at least 6 hours.
Let stand at room temperature about 20 minutes before serving.
• Mousse can be chilled up to 1 day.
• To vary the flavor, you can replace the 1 teaspoon vanilla with 2 teaspoons instant-espresso powder (dissolve it in the hot cream) or 3 tablespoons Grand Marnier or 2 tablespoons Cognac (either one whisked into strained custard).