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 (mixture will be thick).

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 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 third of cream into chocolate custard to lighten, then fold in remaining cream gently but thoroughly.

In a small saucepan over medium heat, bring sugar and 1/3 cup water to a boil.

Boil until syrup reaches soft-ball stage (238 degrees.on a candy thermometer). In an electric mixer fitted with the whisk attachment, beat egg whites on low until foamy.

Add cream of tartar; beat on medium high until stiff but not dry.

With mixer running, pour syrup down side of bowl into egg whites in a steady stream, and beat on high speed until steam is no longer visible and bowl begins to feel cool to the touch, about 3-5 minutes.

With the mixer still running, beat in butter a few pieces at a time. Add vanilla and beat until smooth. If it looks curdled at any point, keep beating to smooth out.