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.