Combine the grand marnier and orange juice and dip half the sponge fingers quickly in the mixture, do not allow to soak, then lay on the bottom of a square flat dish.
In a heatproof bowl over a pan of simmering water whisk the egg yolks and the sugar, remove from the heat and beat until pale, doubled in volume, creamy and pale.
In a separate bowl whisk the cream until thick. Gradually add the mascarpone cheese to the beaten yolks folding it in carefully with a wooden spoon and then add the vanilla essence.
Fold in the cream one third at a time, carefully. Pour half the mix over the soaked sponge fingers and then layer with strawberries. Add another layer of fingers and then cover with the rest of cheese mix, top with the strawberries and then with another layer of sponge fingers.
Cover with cling film and refrigerate for 2 hours. Remove cling film, and then completely coat the top of the tiramisu with the icing sugar.