Arrange the sponge cake slices in the bottom of a large trifle dish or glass bowl. Spread a thin layer of jam over the cake, and drizzle with sherry or juice if using.
Spoon a generous layer of vanilla custard over the cake. Use a spatula to spread it evenly to the edges of the dish.
Scatter a mix of berries over the custard. You can press some against the side of the dish for a beautiful visual effect.
In a bowl, beat the heavy cream with powdered sugar and vanilla extract until soft peaks form. Spoon or pipe the whipped cream on top of the berry layer.
Repeat the layers once more if your dish is tall enough, finishing with whipped cream on top.
Cover and chill the trifle in the refrigerator for at least 2 hours (or overnight) to let the flavors meld.