In a small saucepan, sprinkle the gelatin over the water and let it soften for 2 minutes.
Add the milk and the espresso powder to the gelatin, and heat the mixture over moderate heat, whisking constantly, until the powder is dissolved. Remove the pan from the heat, stir in the vanilla, and set the pan in a bowl of ice and cold water, stirring the mixture every few minutes until it is thick and cold.
In a small bowl, beat the cream until it just holds stiff peaks and fold the coffee mixture into it gently but thoroughly. Spoon the mousse into 4 chilled long-stemmed glasses and chill it until ready to serve.