Simmer 750g of frozen mixed berries with 200g of superfine sugar and 1 liter of water until the fruit is soft, lifting out some of the fruit and juice partway through and setting it aside.
Soak 9 sheets of gelatin in cold water until soft. Mash the fruit left in the pan and strain the syrup through a sieve, leaving the seeds and skins behind.
Squeeze out the gelatin and stir it into the hot syrup until dissolved. Chill until nearly set.
For the custard, whisk 75g of custard powder and 50g of superfine sugar into 1.2 liters of milk in a saucepan. Cook, stirring, until very thick, then cover the surface and let cool.
Cut 1½ Madeira cakes into cubes and pile them into a large glass bowl. Spoon over the reserved fruit and juice and 4 tablespoons of sherry.
Spread the custard thickly over the cake, pressing it against the sides of the bowl, and leave until set with a skin. Spoon the nearly set jelly over the custard and chill until set.
Whip 300ml of heavy cream with 1 teaspoon of vanilla extract and 2 tablespoons of icing sugar until it just holds its shape. Spread over the jelly, scatter with 2 crushed amaretti cookies, and chill.
