Aardbeien-roomkoekjes

Pale, uneven results almost always mean the oven wasn't fully preheated or the pastry was baked below 190°C. Folding the sheet in half lengthwise gives a double layer that needs the full 15-18 minutes to set and colour through, so pull them only when they're deeply golden, not just light brown. Also brush the egg yolk and milk wash only on the tops - if it runs down the cut edges it glues the layers together and blocks the rise.

Full-fat cream cheese is the closest substitute and keeps the filling pipeable, though it tastes tangier than mascarpone, so taste before adding all 40 g of powdered sugar. Whipped ricotta works too if you drain and blend it smooth first, but it's grainier. Avoid low-fat or spreadable tub cheeses - they thin out the whipped cream and the filling will slump between the pastry halves.

Bake the pastry circles up to a day ahead and keep them uncovered or loosely covered at room temperature so they stay crisp - never in the fridge, where they go soft. The mascarpone cream can be made a day in advance and kept in the piping bag in the fridge; give it a gentle squeeze to loosen before piping. Slice the strawberries and assemble only just before serving, since the jam and fruit start softening the pastry immediately.

Assembled sandwiches keep in an airtight container in the fridge for about a day, but the pastry loses its crispness once the cream and jam sit against it. If you know you'll have leftovers, store the baked circles, cream and sliced berries separately and build them fresh. Don't freeze the filled sandwiches - the whipped cream and mascarpone separate on thawing.

Two things cause this: the heavy cream wasn't whipped to stiff peaks, or the mascarpone was overworked once the cream was folded in. Use the cream straight from the fridge, whip it stiff with the powdered sugar first, then fold it into the smooth vanilla mascarpone with a spatula in just a few strokes. If the pastries are still warm when you slice and fill them, the cream will melt too, so let them cool completely.