In a small cup, mix together the cocoa and sugar. Add the water and stir well until a paste is formed. Put the milk in a tall fountain glass. Slowly add the seltzer water. Stir gently. Drizzle the chocolate syrup in slowly, being careful to disturb the foam as little as possible. Stir gently with a tall spoon. Your drink should be two-toned, chocolate brown on the bottom with white foam on top. Serve with a pretzel rod.