In a saucepan, stir together the sugar and cocoa to get rid of lumps; stir in the water until smooth, then whisk in the milk over medium-high heat. Heat until steaming. Remove the pan from the heat and add the peanut butter and chocolate. Let it sit for a few seconds, then blend it with a hand-held immersion blender. Or for individual cups, divide the chocolate among the cups and pour the warm milk overtop. Whiz with one of those little frothers to make it very smooth and foamy.