Using a hand mixer or stand mixer fitted with a whisk attachment, whisk egg whites on low speed until frothy. Add salt, turn up the speed to medium. Slowly add sugar and turn the speed to high. Whisk until stiff peak is form and the mixture looks shiny.

Fold in egg yolks into the mixture with a spatula until well combined. Sift in flour and matcha. Fold until well combined.

Pour the mixture onto the baking pan and spread evenly using an offset spatula. Bake for 10-15 minutes or until the top springs back when lightly pressed.

Let the sponge cake cool in the pan on a rack. Set aside.

While waiting for the sponge cake to cook, add matcha and sugar into a small bowl.

Add little bit of hot water into the matcha and mix until well incorporated. This will prevent the matcha forming too much lumps. Then, add the remaining hot water to the matcha mixture until well combined.

Let the mixture cool completely, then add rum and stir until well combined.

In a heatproof bowl that rests easily in your saucepan without touching the surface of the water (bain-marie), whisk together the egg yolks and sugar until the mixture is pale, thick, and hot to the touch. Make sure the sugar is fully dissolved. Set aside to cool for about 15 minutes.

Meanwhile, whisk heavy cream using an electric mixer until stiff peaks form.

Once egg yolk mixture has cooled, add the mascarpone, rum, and sift in matcha powder. Whisk the mixture until well combined. Then, gently fold in the whipped cream in until smooth.

In a 2.5x3-inch/6x7.5 cm glass, put in a cut out sponge cake. Spoon matcha syrup over the sponge cake, making sure it is soaked with the syrup. Spoon a dollop of the mascarpone cream mixture on top.

Repeat with layering filling and ladyfingers, creating 3 layers in total. 

Cover and refrigerate for at least 6 hours or overnight.

Right before serving, dust the top with matcha powder. Serve cold.