In a small saucepan, bring the bay leaf, cinnamon stick, and 2 cups water to a boil, reduce the heat to maintain a simmer, and cook for 15 minutes. Stir in the grated cocoa, cook for 10 more minutes, then stir in the milk and sugar. Stir in the cornstarch slurry and cook until thickened, about 2 minutes. Remove the cocoa from the heat, stir in the vanilla, then pour through a fine sieve, and discard the solids. Serve the cocoa in warm mugs.