Dissolve mocha ingredients together and set aside to cool:
3 tsp coffee granules
2 tsp cocoa powder
1 cup boiling water
Whisk until mixture is pale creamy yellow and leaves a ribbon trail when whisk is lifted out:
5 egg yolks
170g caster sugar
Add 500g mascarpone and beat until smooth.
In a separate bowl, whisk 5 egg whites (ensure the bowl and all equipment is dry and grease-free) until soft peaks form. Fold into mascarpone mixture.
Pour cold mocha into a shallow dish to start soaking the sponge fingers. You will need about 36 sponge fingers (also known as savoiardi or ladyfingers), in total for the whole tiramisu. Dip enough, a few at a time, to line the base of an 8" x 11" casserole dish. Only soak them briefly- the biscuits should still be firm enough to hold up without breaking- and press them into a tightly packed layer in the base of the dish.
Spread half the mascarpone mixture over, then top with another layer of soaked sponge fingers. Layer over the remaining mascarpone and dust lots of unsweetened cocoa powder over the top to finish.
Refrigerate for at least a few hours, or overnight for a better flavour.