Mandarinlommer

Usually there is too much filling or the corners were not pressed together firmly enough. Stick to about 1 tablespoon per square and press the four folded corners down into the filling so they stick. It also helps if the cut from each corner stops before the center, so the pastry still holds together as one piece.

Yes - any thick, unsweetened fresh cheese works, such as mascarpone or well-drained ricotta. What matters is that it is firm rather than runny, because a wet filling will soak the puff pastry from below. If your substitute is looser than quark, keep the cornstarch in, since that is what binds the filling as it bakes.

You can, but peel the segments carefully and remove the thin skins, since they stay chewy in the oven. Fresh segments also carry more free juice, so blot them just as thoroughly as the canned ones before chopping. The recipe already insists on very well drained, patted-dry mandarins for exactly this reason.

Mix the quark filling a few hours ahead and keep it covered in the fridge, then stir it briefly before spooning it in. Shaping the pockets far ahead is less successful, because the moist filling softens the raw pastry the longer it sits. If you do assemble early, chill the tray and brush with the egg yolk and milk only just before it goes into the oven.

Because of the quark filling they belong in the fridge, loosely covered rather than in an airtight box, which would trap steam and soften the pastry. They taste best on the day they are baked. To bring back the crispness, warm them in the oven for a few minutes at the same 180°C (350°F) and dust with powdered sugar afterwards, not before.