Stir the cold water into the gelatin until dissolved, then let stand for 5 minutes.  The gelatin will absorb the water and begin to set (called "blooming").

Pour the maple syrup into a small saucepan over medium heat.  Let the syrup warm up for a minute, then whisk in 1 tablespoon of the gelatine until dissolved.  For an extra firm mousse, use all of the gelatine.

Bring the syrup to a gentle simmer, then very slowly pour it into the egg yolks, whisking them continuously as you pour.  Dump the mixture back into the pan and cook over medium heat until the mixture reaches 160 F on a candy thermometer.

Pour the maple syrup mixture through a sieve into a small bowl to remove any lumps.  Set it inside an ice bath for 10 minutes, stirring occasionally, until the liquid has thickened to a syrupy consistency and has cooled to room temperature.

Meanwhile, beat the whipping cream until stiff peaks form.  The cream should be thick enough not to fall out of the bowl when it's turned upside down.

Pour the maple syrup mixture into a large bowl, then stir two large spoonfuls of whipped cream into it.  Add the rest of the whipped cream and gently fold it in with a silicone spatula until no lumps or white streaks remain.

Pour the mousse into glasses or bowls and chill in the fridge for at least 2 hours.  Garnish with caramelized walnuts and serve immediately.