Let's cook.
In a small bowl, whisk together the matcha powder and hot water until smooth and no lumps remain. Set aside to cool slightly.
Melt the white chocolate in a heatproof bowl set over a saucepan of simmering water (double boiler method), stirring until smooth. Alternatively, microwave in 30-second intervals, stirring after each, until melted. Let it cool slightly.
In a large bowl, combine the cold heavy cream, granulated sugar, and vanilla extract. Beat with an electric mixer on medium-high speed until soft peaks form, about 3-5 minutes.
Gently fold the cooled melted white chocolate into the whipped cream until just combined.
Carefully fold in the matcha mixture until the mousse is uniformly green and streak-free. Be careful not to overmix.
Divide the mousse evenly among 4 small serving glasses or ramekins.
Refrigerate for at least 2 hours, or until thoroughly chilled and set.
Serve chilled, optionally garnished with a dusting of extra matcha powder or a few white chocolate shavings.
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