Break up the trifle sponges and place them in a large glass serving dish. Crumble the amaretti cookies over them and then sprinkle with amaretto or sweet sherry.
Squeeze the juice from 2 of the mandarins and sprinkle into the dish. Segment the rest and out them in the dish.
Make the custard. Melt chocolate in a heatproof bowl over hot water. In a Seperate bowl mix the cornstarch, sugar and egg yolks to a smooth paste.
Heat the milk in a small saucepan until almost boiling then pour in a steady stream onto the egg yolk mixture, stirring constantly. Return to the clean pan and stir over low heat until the custard has thickened slightly and is smooth.
Stir in the mascarpone until melted, then add the melted chocolate, mixing it thoroughly. Spread evenly over the trifle, cool, then chill until set.
To finish, spread the fromage frais over the custard, then decorate with chocolate shapes and remaining mandarin slices just before serving.