Make the filling: In a mixing bowl, whisk the mascarpone, sour cream and custard powder until smooth.
Whip the cream: In another bowl, whisk heavy cream, sugar,sifted matcha powder, and pandan extract using an electric mixer until SOFT peaks form.
Finish the matcha mascarpone cream: Gently fold the whipped cream into the mascarpone mixture until fully incorporated, then set aside.
Make the matcha soak: In a wide bowl, whisk together hot water, matcha powder, and caster sugar until the sugar has dissolved and the mixture is smooth and lump-free.
Assemble the base: Prepare an 8x8-inch baking dish. Working one at a time, dip the ladyfingers quickly into the matcha soak, coating both sides without letting them get soggy, then arrange them in a single, even layer in the dish.
Spread evenly a thick layer of the mascarpone mixture on top using a spatula.
Make layers: Add another layer of soaked ladyfingers, then spread a layer of mascarpone mixture on top. Smooth the surface with a spatula, cover the dish, and refrigerate for at least 4–5 hours to set (I keep it overnight).
Serve: Just before serving, sprinkle matcha powder over the tiramisu and enjoy.
Notes
Store the tiramisu in the fridge, covered, for up to 3 days.