Preheat the oven to 350°F (175°C). In a mixing bowl, combine flour, sugar, and baking powder. Add melted butter, eggs, and vanilla, and mix until smooth.

Pour the batter into a greased baking pan and bake for 25-30 minutes or until a toothpick comes out clean. Let cool.

In a saucepan, whisk together milk, sugar, cornstarch, and salt. Cook over medium heat, stirring constantly until thickened.

Remove from heat and whisk in egg yolks and vanilla. Let cool.

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

In trifle cups, layer pieces of sponge cake, custard, mixed berries, and whipped cream. Repeat layers until cups are filled.

Chill in the refrigerator for at least 1 hour before serving.