Start by warming up the milk in a saucepan until it's hot but not boiling.

In a medium-sized pan, whisk together egg yolks, sugar, and corn starch until light and fluffy. Gradually incorporate the warm milk while continuing to whisk vigorously to avoid lumps forming in your mixture.

Place the pan over medium heat and stir continuously until it reaches a slow boil, then lower the heat and allow it to cook for another couple of minutes until you've reached your desired thickness before transferring it to a glass bowl.

Preheat your oven to 400F (200C). Roll out your puff pastry on a lightly floured surface and cut it into squares.

Using a sharp knife, carefully cut a half-inch edge around each square while leaving two opposite corners uncut. Fold these cut corners over towards one another, prick the center part with a fork, then brush edges with milk or an egg wash before placing them in the oven.

Once baked and cooled slightly, whisk your custard until smooth again before piping or spooning it into each pastry shell. Top generously with fresh berries and almond slices; finish off by dusting with powdered sugar if desired before serving.