Cook 1/4 cup sugar in a heavy saucepan, 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
Melt chocolate in a metal bowl set over a saucepan of barely simmering water, stirring, and remove from heat
Using an electric mixer beat egg yolks with remaining 3/4 cup sugar until thick and pale, in another bowl
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 degree 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.
