Put the custard into a pan with the chocolate pieces. Gently heat, stirring, until the chocolate has melted into the custard. Cover with cling film and cool.
Arrange the brownies in the base of a trifle bowl. Drain the jars of cherries, reserving the liquid, and scatter over the brownies. Drizzle over 100ml reserved liquid. Spoon the cooled chocolate custard over and chill while you make the topping.
Lightly whip the cream with the crème fraîche and icing sugar until soft peaks form. Chill until ready to serve, then pile on top of the trifle and decorate with grated chocolate and fresh cherries, if you like.