1 cup sugar
2 cups milk
1 cup cocoa powder, sifted
3 1/2 ounces bittersweet chocolate, chopped
4 egg yolks, beaten lightly
In a heavy saucepan cook 1/4 cup sugar, undisturbed, over moderate heat until it begins to melt and cook, stirring with fork, until melted completely and deep golden brown. Remove pan from heat and dip pan briefly into a bowl of ice water to stop cooking. (Caramel will harden). Cool pan about 5 minutes and return to heat. Add milk and cook over moderate heat, whisking until caramel is melted. Whisk in cocoa until combined well and keep mixture warm.
In a metal bowl set over a saucepan of barely simmering water melt chocolate, stirring, and remove from heat. In a bowl with an electric mixer beat egg yolks with remaining 3/4 cup sugar until thick and pale. Whisk in caramel mixture and chocolate in streams, whisking until combined. Pour custard into another 3 quart heavy saucepan and cook over moderately low heat until a candy thermometer registers 140 F. Cook custard stirring (do not let it boil), 4 minutes more and remove from heat. Cool custard completely and freeze in an ice cream maker.