For the custard: Mix together sugar and cornstarch in a large saucepan. Add egg yolks, and whisk to combine; then gradually whisk in milk. Cook over medium-low heat, stirring constantly, until custard thickens to the consistency of thick cream, about 25 minutes. Transfer to a large bowl, and add vanilla. Cover surface of custard with plastic wrap to prevent a skin from forming, and refrigerate until completely chilled, about 2 hours.
For the trifle: Cut cake into 2'' X 1'' pieces. Arrange a layer of cake pieces in the bottom of a large trifle or glass bowl. Sprinkle cake with some of the marsala; then spread a layer of strawberry jam over the cake; then scatter some of the raspberries over the jam. Pour some of the custard over the berries. Repeat layering, ending with custard. Cover with plastic wrap, and refrigerate for at least 3 hours. Remove trifle from refrigerator about 1 hour before serving.
Just before serving, put heavy cream and sugar into a large, well-chilled mixing bowl. Beat cream with a whisk or an electric mixer fitted with whisks until cream holds soft peaks. Do not overbeat. Decorate trifle with large dollops of whipped cream.