Combine mascarpone cheese and sugar in a bowl and mix well.
Add heavy cream and whisk until stiff peaks form.
Mix matcha powder with hot water to make matcha syrup.
Soak castella cake or ladyfingers in matcha syrup and lay them on the bottom of a mold.
Spread a layer of cheese cream on top.
Sift matcha powder over the top.
Repeat the layering process 2–3 times.
Refrigerate for at least 2 hours.