In a medium heat-proof bowl, combine coffee, water gelatin and sugar. Place bowl over a pot of gently boiling water and stir until the sugar is melted. Watch carefully to ensure that the mixture doesn't come to a boil, or the texture of the gelatin will get messed up.
Place bowl into a bigger bowl or pan of ice water to stop cooking and quickly bring mixture to room temperature. Stir to help cool it, then set mixture aside.
In a large bowl, use an electric mixer fitted with a beater/whisk attachment to beat the whip until stiff peaks form.
With the mixer on low, slowly add the coffee mixture, vanilla and salt and beat until incorporated and mixture has reached soft peaks.
Spoon or pipe the mousse into small glasses, then top with desired garnishes. Place in fridge to firm up a little more, about 10-20 minutes.
Store in fridge until to ready to serve.
Recipe by Overtime Cook at https://overtimecook.com/2019/04/11/egg-free-coffee-mousse-for-passover/