Cut the sponge cake into cubes and soak them in sherry if using.

In a trifle dish, layer half of the soaked sponge cake cubes at the bottom.

Spread half of the custard over the sponge cake.

Add a layer of mixed berries on top of the custard.

Repeat the layers with the remaining sponge cake, custard, and berries.

Whip the heavy cream with powdered sugar until soft peaks form.

Spread the whipped cream over the top layer of berries.

Refrigerate the trifle for at least 2 hours before serving to allow the flavors to meld.