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The most common cause is dough that was too wet or too cool when piped: after adding the flour, keep stirring until the dough forms a ball and leaves a light film on the pan, then cool it for 5 minutes before working in the egg. If the batter runs when piped instead of holding a 3.5 cm mound, it has taken too much egg - add the egg gradually and stop while the dough still falls in a thick ribbon. Opening the oven during the 25-30 minutes also collapses shells before their structure sets.

Yes - the 3 tbsp of ground hazelnuts are stirred into the finished pastry cream, so ground almonds or ground pistachios swap in one-for-one without changing the cooking method. If you swap them, consider leaving the ½ tsp cocoa out of the craquelin for a paler top that matches the new flavour. For a nut-free version, skip the nuts entirely and lean on the vanilla pod, which already flavours the milk.

The craquelin is designed for advance work: it needs at least 1 hour rolled out between parchment and refrigerated, and it can rest there longer or go into the freezer until you cut the 4 cm circles. The hazelnut cream can also be cooked ahead - cover it directly on the surface so no skin forms, then chill it and whisk it smooth before filling. Assemble only shortly before serving so the shells stay crisp.

Once filled with the milk-and-egg-yolk cream, the choux must be refrigerated, and they are at their best the same day. The cream softens the shells over time, so leftovers turn tender rather than crisp - still good, just no longer crunchy. Store unfilled shells separately at room temperature and dust with powdered sugar only right before serving, since it dissolves on contact.

Lumps usually mean the hot milk went into the yolk mixture too fast: pour it in gradually while whisking so the yolks temper instead of scrambling. Raw-starch flavour means the 35 g of cornstarch was not cooked out - keep stirring after it thickens so the starch cooks through. If lumps still appear, pass the cream through a sieve before stirring in the ground hazelnuts.