For Hens: Halve the hens through the breast. Flatten the breasts with the palm of your hand.
In a mortar or food processor, pound or mince the garlic, shallots and sugar. Add the salt, pepper, five-spice powder, fish sauce, soy sauce and sherry. Mix thoroughly. Pour the mixture over the hens. Cover and refrigerate for at least 2 hours or overnight.
Prepare a fire in a barbecue. Let the coals burn down to medium-hot. Set the hens skin-side down on the rack and grill over medium-hot coals for 15 minutes. Turn and grill for 15 minutes longer, or until they are thoroughly cooked and the skin is crisp and browned. Serve with Nuoc Cham Dipping Sauce.
For Dipping Sauce: In a mortar, pound the garlic, chile pepper and sugar into a paste. (Or blend in a food processor or blender.) Transfer the mixture to a bowl and add the fish sauce, lime juice and water; stir to blend. Let the mixture sit for 5 minutes, or until ready to serve. Strain into dipping saucers. Yields about 3/4 cup.