In a small saucepan over medium heat, combine the apple cubes with 200 ml apple juice, a squeeze of lemon juice, some grated lemon zest, a pinch of vanilla sugar, and some sugar.
Bring to a simmer and cook for 5 minutes, stirring frequently.
In a small bowl, mix some pudding powder with the remaining 100 ml apple juice until smooth.
After the 5 minutes are up, add the pudding mixture to the simmering apples and stir vigorously with a whisk. Bring to a boil over medium-high heat for 1 minute, then remove from heat.
Stir occasionally as the mixture cools to lukewarm, about 10 minutes.
Preheat the oven to 220 °C (425 °F) on top and bottom heat.
Remove the puff pastry sheets from the freezer and cut into squares using a pizza cutter or sharp knife. Lay on a work surface to thaw for about 5 minutes.
Stir some mascarpone into the cooled apple mixture.
Place roughly 1/2 to 1 tablespoon of the apple-mascarpone filling slightly diagonally in the center of a pastry square.
Fold the pastry diagonally in half to form a triangle.
Press the edges together firmly to seal, preventing any filling from leaking.
Transfer to a baking sheet and repeat with the remaining pastry squares and filling.
In a small bowl, whisk together 1 egg with a splash of milk.
Brush the egg-milk mixture over each pastry triangle and poke holes across the top with a fork.
Bake for 15 to 20 minutes until golden brown.
Serve warm from the oven, or let cool completely, refrigerate for 1 to 2 hours, and serve chilled.
The Boss Kitchen editorial staff oversees content review, fact-checking, and recipe verification across the site. Published articles pass through the editorial team before going live, ensuring ingredient lists, techniques, cooking times, and nutritional claims hold up in a home kitchen. The team coordinates contributions across the site writers, handles reader corrections, and maintains consistency in measurement conventions, safety guidance, and dietary labeling. Posts under this byline typically represent team-reviewed reference material, site announcements, or editorial roundups rather than individual-author features, and they are held to the same sourcing standards as bylined recipe and product coverage.