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 bowl, combine mixed berries and granulated sugar. Let sit for about 10 minutes to release juices.

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

In a trifle bowl, layer half of the sponge cake, followed by half of the custard, half of the macerated berries, and half of the whipped cream. Repeat the layers.

Top with additional berries and a dollop of whipped cream. Chill for at least 1 hour before serving.