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In a medium saucepan, whisk together the cocoa, sugar, ClearJel or cornstarch, and salt.
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Slowly whisk in the water, then the flavor of your choice and the milk.
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Bring to a boil over medium heat, whisking frequently as the mixture warms, then almost constantly as it becomes hot; you don't want the mixture to stick to the bottom of the pan.
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Remove from the heat and stir in the chopped chocolate or chocolate chips, whisking until chocolate melts.
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Pour into a bowl, and stir occasionally as it cools, to prevent a skin from forming.
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Lay a piece of plastic wrap on the surface of the chocolate, again to prevent a skin from forming, and refrigerate until it's well chilled; overnight is good. Make sure the canister for your ice cream maker is in the freezer, too; it needs to be as cold as it can be.
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Next day, pour the chocolate into the ice cream maker, and freeze for 20 to 25 minutes, until it's quite stiff.
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Scoop the ice cream into a bowl. For best texture, stir in 1/4 cup coffee liqueur (e.g., Kahlua), or the liqueur of your choice. This will keep the ice cream soft and scoopable indefinitely in the freezer.
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Serve the ice cream immediately, if desired; it will be very soft. For harder ice cream, store in the freezer. Ice cream without liqueur will become nicely hard in 3 hours. With the addition of liqueur, it will take 6 hours or more to become quite solid. Ice cream without liqueur will become unpleasantly hard within 5 hours or so; to soften, let sit at room temperature for 30 minutes or so before serving.