Mix the egg yolk and three tablespoons of the sugar in small mixing bowl and set it next to the stove.

In a large saucepan, combine remaining sugar, cornstarch, cold milk, salt and coffee crystals. Turn heat to medium and bring to a simmer. Gradually pour about a cup of the hot milk mixture into the egg yolk, stirring constantly, then pour the egg mixture into the saucepan, stirring constantly. Continue stirring the mixture until it starts to thicken (almost as thick as gravy, not as thick as pudding) and reaches a temperature of 183F on a candy thermometer.

Pour the hot custard mixture into a 13x9 inch glass pan. Gradually pour in the 1 cup of cold heavy cream, then stir in the vanilla. Put the mixture in the refrigerator for about 2 hours or until it is thoroughly chilled.

When chilled, pour into your ice cream maker and process as directed by the manufacturer. During the last 60 seconds, drop gobs of fudge sauce into the ice cream mixture so that it’s ripped through and not mixed in. Add some brownie pieces and nuts if desired. Transfer to a covered container and freeze until it is firm enough to serve.