In large heavy saucepan whisk cocoa powder, granulated sugar, cornstarch, and 1/4 tsp. salt. Add 1/2 cup of the milk; whisk to a smooth paste. Whisk in remaining milk and whipping cream. Stir constantly over medium heat until pudding thickens and begins to bubble at edges. Stir 30 seconds more. Add chocolate; stir 1 minute to melt. Remove from heat; stir in rum and vanilla.
Transfer pudding to lightly oiled 9-inch deep-dish pie plate. Cook, uncovered, for 2 hours at room temperature. Cover and refrigerate overnight.
For cinnamon toasts, in small bowl combine brown sugar, butter, cinnamon, and 1/8 tsp. salt. Spread on one side of each baguette slice. Place on baking sheet. Broil 5 to 6 inches from heat for 2 to 3 minutes or until toasted and sugar mixture is bubbly.
Sprinkle pudding with additional cocoa powder. To serve, cut in wedges or, with a large spoon, spoon in free-form wedges. Serve with cinnamon toasts and grapes. Makes 12 servings.