In a large microwave-suitable bowl, dissolve the matchapowder in hot water until it forms a smooth paste.
Add the white chocolate and cream, and place in the microwave.
Microwave on a low-medium heat for 30 seconds, then stir.
Repeat this until everything is evenly melted and combined.
Cover the bowl and chill in the fridge for 30 minutes - 1 hour, until set but still soft enough to whip.
Using a handheld electric mixer, beat the mousse until fluffy
Spoon into serving glasses and serve or set aside in the fridge.