Start by sprinkling gelatine over the water in a saucepan. Warm it gently on low heat while stirring until fully dissolved. Avoid boiling, as high heat can weaken the gelatine.
Stir instant coffee and sugar into the warm gelatine mixture. Keep stirring until everything is completely dissolved and smooth. Place the mixture in the fridge until it cools but remains pourable.
Pour cold whipping cream into a clean bowl and whip until soft peaks form. The cream should be fluffy but not stiff or grainy.
Add vanilla extract to the cooled coffee mixture. Mix in a small portion of whipped cream first to loosen it, then gently fold in the remaining cream in batches.
Spoon the mousse into serving glasses. Refrigerate for at least two hours until set. Garnish just before serving for the best look.