Make the custard cream: Begin by warming up the milk in a medium saucepan over medium heat until it’s hot but not boiling. In another bowl, whisk the egg yolks with the sugar and corn starch until the mixture is light and fluffy. Gradually pour in the warm milk while whisking, ensuring there are no lumps.
Place this mixture back into the pan and heat it over medium heat while stirring continuously. Keep stirring until it reaches a slow boil and thickens nicely. Lower the heat and let it cook for a couple more minutes. Don’t forget to add the vanilla extract, give it a whisk, and let it cool, covering it with plastic wrap.
Make the pastries: Preheat your oven to 400°F (200°C). Roll out the puff pastry and cut it into squares, about 5 inches on each side. Carefully cut a half-inch border around each square into a two L shape, but remember not to cut through the opposite corners. Fold those corners over to the center, prick the middle with a fork, and brush the edges with a little milk or egg wash. Bake these beauties for 15-18 minutes until they are golden and puffed.
Finally, whisk the custard until it’s lovely and smooth. Now the fun part! Fill each puff pastry with that smooth custard, top with delightful berries and almond slices, and finish with a light dusting of powdered sugar before serving.