In a saucepan, whisk together milk, sugar, cornstarch, and egg yolks. Cook over medium heat, stirring constantly until thickened. Remove from heat and stir in vanilla extract. Let cool.

In a mixing bowl, beat the heavy cream, powdered sugar, and vanilla extract until soft peaks form.

In a trifle dish, layer half of the sponge cake cubes, half of the custard, half of the mixed berries, and half of the whipped cream. Repeat the layers with the remaining ingredients.

Finish with a layer of whipped cream and garnish with additional berries on top.

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