In the top of a double boiler, whisk together sugar, cocoa, corn starch, salt, and cinnamon. Whisk in milk and egg.

Place over boiling water and cook, whisking continuously, until thickened, about 7 minutes. Continue cooking, while whisking, for three minutes to ensure the pudding is fully thickened. Remove from heat.

Stir in chocolate, butter, and vanilla, until melted and smooth.

Pour into serving bowls and chill, optionally placing plastic wrap on the surface to prevent the formation of a skin.