In 1- to 1 1/2-quart pan, stir together gelatin, sugar, 1/4 cup of the cocoa, and water. Bring to a boil over high heat, stirring constantly. Pour into a deep 2-quart metal bowl and stir in milk. Cover and freeze until firm (about 45 minutes).
Beat frozen mixture with an electric mixer on high speed until fluffy and thick enough to hold peaks (about 3 minutes); stir in vanilla and amaretto, if desired. Divide mousse among four 1/2-cup ramekins or bowls; smooth tops. If made ahead, cover (do not let wrapping touch mousse) and refrigerate for up to 1 day. Dust with remaining cocoa before serving.