Caffe Gelato

2/3 cup sugar
4  egg yolks
1 cup milk; at room temperature
1/8 tsp salt
4 tsp instant coffee
1 cup heavy cream

Beat the sugar and egg yolks together until pale yellow and very thick. Slowly add the milk, beating gently to avoid a build-up of foam. Stir in the salt.  Transfer mixture to the top of a double boiler with an inch of boiling water in the bottom half, or into a or a metal bowl set over a saucepan of barely boiling water. Regulate heat so the water remains at a low boil, and stir continuously for 8 minutes. The custard will thicken enough to coat the spoon, and surface foam will disappear. Immediately remove top of double boiler and set it in a large bowl of cold water. Stir 2 minutes to cool custard somewhat. Then transfer it to a non-metallic bowl and add the coffee, stirring to dissolve it thoroughly. Cover and refrigerate to chill thoroughly. Also chill heavy cream. When ready to proceed, whip cream into soft peaks and stir it into coffee custard. Transfer mixture to ice cream machine and proceed according to machine's directions.