Dissolve matcha powder in 1/4 cup hot water then add sugar and 3/4 cup cold water. Chill in the fridge while making the filling.
Beat mascarpone until softened then add whipping cream and beat on high speed, gradually adding sugar. Whip until firm peaks form.
Assembly:
Spread about a tablespoon of the filling at the bottom of a 6x6 pan.
Dip both sides of lady fingers into the matcha quickly, about a second on each side so as no to soak it too much.
Arrange a layer of ladyfingers onto the bottom of the pan, add half of the mascarpone-cream filling on top, sprinkle a little matcha powder then repeat the steps to make another layer.
Chill for at least 4 hours, Overnight or longer is better.