In a small bowl, sprinkle the gelatin over 1/4 cup cold water and let it sit for 5 minutes to bloom.
Add the hot water to the gelatin mixture and stir until completely dissolved.
In a separate bowl, whisk together the sugar, espresso powder, 1/2 cup heavy cream, and vanilla extract.
Stir in the dissolved gelatin mixture until fully combined.
In a large bowl, beat the remaining 1 cup heavy cream until stiff peaks form.
Gently fold the whipped cream into the coffee mixture until smooth and fluffy.
Spoon the mousse into individual serving cups and refrigerate for at least 2 hours before serving.