In a small saucepan, heat the milk, sugar, and matcha powder over medium heat for about 5 minutes until the sugar is dissolved and the mixture is heated through, stirring constantly. Don’t allow it to boil, just gently heat it until it’s steamy. Then, set the pan aside, off the heat.
Strain any undissolved matcha and collect the matcha milk in a separate bowl. Set aside.
In a small, stainless steel bowl, pour the Knox powdered gelatin over the cold water. Let stand for 5 minutes to bloom.
While the gelatin is blooming, heat a saucepan of water to a gentle simmer, then place the bowl of bloomed gelatin over the simmering water to gently melt the gelatin, stirring constantly. The solution will go from cloudy to clear.
Take the bowl of gelatin off the heat and whisk it into the milk mixture. Set aside to cool slightly.
Whip the cream to stiff peaks, but not granular (don’t make butter!).
Carefully pour the tepid matcha milk into the whipped cream, and fold to incorporate it. (Don’t be afraid to really mix it well and get out all the lumps.)
Your mousse is ready to be poured over a cake base and chilled overnight (remember to prepare the cake base before making the mousse!).