Halve each slice of sponge cake and use twenty of them for the Tiramisu. Dissolve gelatine in three tablespoons of hot water. Whip rich cream with powdered sugar till stiff and set aside.
Whip together egg yolks with sugar and one tablespoon of water over a double boiler till the mixture reaches a ribbon stage. Mix two tablespoons of instant coffee powder in half a cup of water and soak the chocolate sponge fingers in this solution.
Arrange half of them in a layer at the base of a spring bottom tin. Mix two teaspoons of instant coffee powder in one teaspoon of water. Add this along with mascarpone cheese and gelatine to the egg mixture and mix well.
Fold in the whipped cream. Pour half this mixture over the chocolate fingers. Arrange the remaining chocolate fingers over this and then top up with the remaining mixture and level. Place it in the refrigerator till well set. Cut into wedges and serve chilled.