Soak the gelatin in the cold water for at least 3 minutes. Heat the milk in a saucepan over medium heat just until bubbles begin to form around the edge. Turn the heat as low as it will go, and add the cocoa and the sugar. Stir until they dissolve. Add the gelatin to the milk mixture, and stir until the gelatin also dissolved. Refrigerate until set, at least 3 hours. Once the gelatin has set completely, put it in a blender with 2 tablespoons of the milk. Blend until the mixture has a pudding-like consistency. If necessary, add more milk. It's best if you serve it right away, but if you have to wait, just give it a quick blend just beforehand to mix in any ingredients that may have separated out.