In a medium saucepan combine the sugar and 1/4 cup of the water and cook over medium-high heat, undisturbed, until the sugar melts and begins to boil. Allow the sugar to cook, swirling occasionally, until the syrup reaches a deep golden caramel, about 5 minutes. Add the water carefully; the caramel will harden and steam vigorously. Cook over low heat until the caramel is dissolved. Add the cocoa powder, cinnamon, cayenne and salt and whisk until dissolved. Transfer to a clean bowl and cool, stirring occasionally. When the mixture has cooled, stir in the vanilla and transfer to the refrigerator until thoroughly chilled.
Freeze in an ice cream maker according to manufacturer's directions. Transfer to an airtight container and freeze for at least two hours before serving.
Yield: about 3 cups