Sprinkle the gelatin over the cold water in a small bowl and let it sit for about 5 minutes.
Combine the cooled espresso, sugar, and instant espresso powder in a small saucepan.
Warm the coffee mixture gently over low heat until the sugar dissolves.
Add the softened gelatin and stir until completely dissolved.
Remove from the heat and stir in the vanilla extract.
Allow the coffee mixture to cool to room temperature.
In a large mixing bowl, whip the heavy cream with the salt until soft peaks form.
If using cocoa powder, sift it into the whipped cream and gently incorporate it.
Slowly fold the cooled coffee mixture into the whipped cream.
Fold gently until the mixture is smooth and airy.
Divide the mousse among six dessert glasses or ramekins.
Cover and refrigerate for at least 2 hours, or until set.
Garnish with whipped cream, chocolate shavings, cocoa powder, or coffee beans.
Serve chilled.