This recipe is really smooth, like a frozen fudgicle.
2 1/4 cups water
1 cup sugar
3/4 cup unsweetened Dutch-process cocoa powder
pinch of salt
6 ounces bittersweet or semi-sweet chocolate, finely chopped
1/2 teaspoon vanilla extract
In a large saucepan, whisk together 1 1/2 cups of the water with the sugar, cocoa powder, and salt. Bring to a boil, whisking frequently. Let it boil, continuing to whisk, for 45 seconds.
Remove from the heat and stir in the chocolate until it’s melted, then stir in the vanilla extract and the remaining 3/4 cup water. Transfer the mixture to a blender and blend for 15 seconds. Chill the mixture thoroughly, then freeze it in your ice cream maker according to the manufacturer’s instructions. (If the mixture has become too thick to pour into your machine, whisk it vigorously to think it out.)
(I believe this recipe is by David Lebovitz, although I found it in various places on the internet.)