Sprinkle the gelatin over the cold water in a small bowl. Let it sit for about 5 minutes to bloom.
In a saucepan, combine the mango puree, sugar, and lemon juice.
Warm the mango mixture gently over low heat. Do not bring it to a boil.
Add the bloomed gelatin and stir until completely dissolved.
Remove from the heat and allow the mango mixture to cool to room temperature.
In a large mixing bowl, whip the heavy cream with vanilla extract and a pinch of salt until soft peaks form.
Gently fold the cooled mango mixture into the whipped cream until smooth and evenly combined.
Spoon the mousse into six dessert glasses or serving cups.
Cover and refrigerate for at least 3 hours, or until the mousse is completely set.
Before serving, garnish with fresh mango cubes, whipped cream, mint leaves, or toasted coconut.
Serve chilled and enjoy.