To make the custard, put the milk in a microwave-proof bowl and heat on 900W for 3 minutes. Meanwhile, in a separate microwave-proof bowl, whisk together the cornflour and sugar. Gradually add the eggs and the vanilla extract, whisking all the while.
When the milk is hot, pour in the egg mixture, whisking continuously. Cook in the microwave on 900W for 3 minutes, stopping the microwave every minute to give the mixture a whisk. It’s ready when the mixture is thick enough to coat the back of a spoon. Set aside and let cool for 10-15 minutes.
Meanwhile, make the trifle. Tip the frozen black forest fruits into a large bowl, reserving a few for decorating. Add the peach slices and icing sugar. Mix well and leave to one side for the frozen fruits to defrost, which will take about 15 minutes.
Arrange the swiss roll slices in the bottom of a glass trifle dish and scatter over the amaretti biscuits. Once the fruit has defrosted, arrange it over the top of the swiss roll, drizzling over the juices in the bottom of the bowl. Pour the cooled custard over the top.
Spoon the whipped cream over the top of the custard and decorate with the reserved black forest fruits and a few extra crushed amaretti biscuits. Dust with a little icing sugar before serving.