For the eggless sponge biscuits
Dissolve the sugar in the milk, add the butter previously softened to room temperature and then sieve the flour and potato starch into this mixture, mixing until it is perfectly even and smooth.
Leave the mixture to rest in the fridge for one hour.
Cover a baking sheet with greaseproof paper and arrange small rectangular portions of the mixture in rows (about 7 cm long and 3 cm wide).
Lightly brush the surface of the biscuits with water and bake in the oven at 190°C for 15 minutes.
For the eggless tiramisu
In a bowl, smoothly blend the mascarpone, cream and sugar. Pour the mixture into a siphon loaded with a cartridge and shake vigorously.
Turn the siphon upside down and place it in the fridge for about one hour. Rapidly soak the sponge biscuits with the cold coffee.
Squirt the frothy mascarpone cream into the bottom of the dish, arrange the coffee-soaked sponge biscuits on top and then repeat with another layer.
Place the bowls in the fridge until you are ready to serve them.
Before serving, sprinkle cocoa powder on top of each bowl of tiramisu.