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The usual causes are dough that was too wet or an oven door opened too soon. Drying the dough over medium heat for about a minute and adding the eggs only after the 10-minute cooling rest keeps the batter thick enough to hold its piped shape. Since the recipe bakes at 200°C (390°F) without opening the oven and then cools the waves inside the switched-off oven with the door closed, any early peek lets steam escape and the shells sink.

Yes - full-fat cream cheese or thick quark works in place of the 250 g mascarpone, though quark is tangier so you may want the full 3 tbsp powdered sugar. Keep the 200 ml whipped cream whipped stiff, because it is what gives the filling enough body to sit between the halves. Vanilla extract can be replaced by the seeds of a vanilla pod or vanilla sugar.

Filled waves belong in the fridge, since the mascarpone-cream filling is perishable. The recipe already notes they can be served right away or chilled until serving - but the longer they sit, the more moisture from the cream and mandarins softens the shells. For the best contrast between crisp shell and soft filling, eat them the same day.

Bake the choux waves ahead and keep them unfilled at room temperature in a loosely covered container; if they lose crispness, a few minutes in a warm oven revives them. The mascarpone cream can also be mixed in advance and kept covered in the fridge. Slice, fill and dust with powdered sugar only shortly before serving, otherwise the sugar dissolves and the shells go soft.

Canned mandarins carry a lot of syrup, which is why the recipe calls for about 175 g well-drained and chopped small. Let them drain in a sieve and pat them dry before folding them in, and fold gently so the whipped cream keeps its volume. If you use fresh mandarins, remove as much of the membrane and juice as you can for the same reason.