Preheat the oven to 350°F (175°C). In a mixing bowl, cream together the softened butter and sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate bowl, combine the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Pour the batter into a greased baking dish and bake for 25-30 minutes or until a toothpick inserted comes out clean. Let cool, then cut into small cubes.

In a saucepan, whisk together the milk, sugar, cornstarch, and salt. Cook over medium heat, stirring constantly until the mixture thickens and comes to a boil.

Remove from heat and whisk in the egg yolks and vanilla extract. Let cool before using.

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

In small glass cups, layer the sponge cake cubes, custard, mixed berries, and whipped cream. Repeat the layers until the cups are full.

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