Equipment: Fondue pot; fondue forks or wooden or metal skewers
In a bowl, whisk together the sugar and cornstarch to blend. Slowly whisk in the cream; then the milk, coffee, cinnamon, and vanilla. Pour the mixture into a stainless steel saucepan and gently bring it to a boil. Cook, whisking, until thickened, about 4 minutes.
Remove the pan from the heat and whisk in the chocolate and butter until melted. Using a fine-meshed sieve over a large bowl, strain the chocolate mixture and discard the solids. Pour into a warmed fondue pot and serve with assorted dippers and fondue forks.