Whisk together the matcha powder, sugar, and hot water until smooth. Set aside to cool.
Whip the heavy cream and powdered sugar until medium-stiff peaks form.
In a separate bowl, mix the mascarpone and vanilla until smooth. Fold into the whipped cream in two additions.
Dip the ladyfingers into the cooled matcha mixture and arrange in an even layer. Spread half the cream on top. Repeat with another layer of ladyfingers and finish with the remaining cream.
Smooth the top, refrigerate for at least 6 hours or overnight, then dust generously with matcha before serving.
