In a small cup, mix together the cocoa powder and sugar. Add the water and stir well until a paste is formed. Set aside.
Put the milk into 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 2-toned, chocolate-brown on the bottom with a white foam on top.