Bake the Sponge Cake:

1. Preheat your oven to 350°F (180°C). Grease and line a square baking pan.
2. In a large bowl, beat the eggs, granulated sugar, vanilla, and salt until thick, pale, and tripled in volume.
3. Sift in the flour and baking powder. Gently fold the dry ingredients into the egg mixture using a spatula to keep the air in the batter.
4. Pour into the pan and bake for 20–25 minutes until golden. Let it cool completely, then cut into bite-sized cubes.

Make the Vanilla Pudding:

1. In a medium saucepan, whisk together the milk, sugar, egg yolks, and cornstarch.
2. Place over medium heat, whisking constantly until the mixture thickens and begins to bubble.
3. Remove from heat and stir in the vanilla sugar and butter until smooth. Press a piece of plastic wrap directly onto the surface of the pudding to prevent a skin from forming and let it cool.

Prepare the Whipped Cream:

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

Assemble the Trifle:

1. In a large glass trifle bowl or individual jars, layer the sponge cake cubes at the bottom.
2. Add a generous layer of the cooled vanilla pudding.
3. Place a layer of sliced strawberries (red) and blueberries (blue) against the glass.
4. Top with the whipped cream layer.
5. Repeat layers if space allows, ending with a decorative swirl of whipped cream and fresh berries on top.