If you want to fill the pastries with custard, make it first and then start on the pastries.

In a small sauce pot, whisk together the cornstarch, sugar, and salt.

Pour in 125 milliliters of milk and whisk to combine. Then pour in the remaining milk, egg yolks, and heavy cream and whisk again.

Place the pot over medium low heat and whisk. Continue to whisk until the mixture just starts to bubble and thicken.

Remove the pot from the heat and pour it through a fine mesh sieve into a heat safe bowl. Then stir in vanilla extract.

Cut the butter into 3 pieces. Then, add in the butter and whisk again until the butter has fully melted.

Lay a piece of plastic over the surface of the custard and set aside to cool. Once slightly cooled place it in the fridge to set completely.

Chop one apple into ½" chunks and remove the core. Place the apple along with the juice of half a lemon and 50 milliliters of water into a small pot.

Place the pot over medium heat and wait until it starts to bubble. Then stir frequently and continue to cook until the apples soften.

In a separate bowl stir together flour, sugar, a touch of salt, and cinnamon.

While stirring the apples, sprinkle in the flour mixture and stir until the liquid thickens.

Remove the pot from the heat and pour into a shallow dish or baking sheet to allow it to cool completely.

Rinse the blackberries and pour them onto a flat plate or a very shallow bowl.

Use a fork to mash the blackberries just until broken down. Set aside.

Preheat the oven to 425 F and place a rack in the center of the oven.

Line a baking sheet, with a sheet of parchment paper.

Make the egg wash by whisking the egg with two tablespoons of water and a sprinkle of salt in a small bowl.

Briefly roll out the sheets of puff pastry just so they are even and smooth.

Cut both sheets of puff pastry into 6 even rectangles.

On the bottom set of six, evenly distribute the apple filling into the centers of the rectangles.

Top the apples with the mashed blackberries.

Brush the edges of the 6 bottom rectangles with the egg wash. Lay the top set of rectangles over the top of the bottom set.

Press the edges of the two sheets of pastry together with your fingers first. Then use a fork to crimp the edges together so they are fully sealed.

Use the same fork to prick the top of each pastry rectangle three times so that steam can escape.

Brush the top of the pastry with more egg wash and sprinkle with turbinado or demerara sugar.

Bake the pastry in the preheated oven for 15 minutes, then rotate and bake for another 15-20 minutes until deeply golden brown.

Once baked, remove the sheet from the oven and place on a wire rack to cool for 10-15 minutes minimum.

To fill with custard, use a pairing knife to cut a small slit in one short end of each pastry. Use a piping bag to pipe ⅙ of the custard into each pastry.