Whisk together the coffee, sugar, cocoa powder, and salt in a large saucepan. Bring the mixture to a boil and allow it to boil for 30 seconds, whisking constantly. Remove from the heat and stir in the milk.
Chill the mixture thoroughly, then freeze it in your ice cream maker according to the manufacturer's instructions.
Perfect Pairing: To make a Mocha Freeze, for each serving put 2 scoops of Mocha Sherbet (4 ounces, 115 g) in a blender along with 1/2 cup (125 ml) very strongly brewed coffee or espresso, 11/2 tablespoons sugar, and 3 ice cubes. Blend until almost smooth. Pour into a glass and top with Whipped Cream.
Reprinted with permission from The Perfect Scoop: Ice Creams, Sorbets, Granitas, and Sweet Accompaniments by David Lebovitz, copyright (C) 2007. Published by Ten Speed Press, a division of Random House.