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Lumps usually come from adding the hot milk too quickly to the egg yolk, sugar and cornstarch mixture, so pour it in slowly while whisking constantly. If it stays runny, the cornstarch simply never reached the temperature it needs to set, so keep cooking and stirring until the custard visibly thickens. A lumpy custard can still be rescued by pushing it through a fine sieve before it cools.

You can, but thaw them completely and pat them very dry first, because the released juice will soak into the dough and make the base soggy. Fresh berries are the safer choice here since they sit directly on the custard with no barrier underneath. Whichever you use, cut larger berries like strawberries into smaller pieces as the recipe says so the braid closes neatly.

The custard is the ideal make-ahead component: cook it, cover it directly on the surface, and chill it - the recipe already requires it to cool completely before use, so a head start costs you nothing. Assemble the strudel only shortly before baking, since the moisture from custard and berries starts softening the dough right away. Keep the croissant dough refrigerated until the moment you roll it out.

The most common cause is overfilling - stick to the 200-250 g of berries and spread the custard in a narrow line down the center, leaving a clear margin on both sides. When you fold the strips alternately over the filling, press the ends down firmly so the braid seals at the top and bottom. Cutting the side strips too short also leaves gaps where the hot custard can escape.

Because of the custard filling, store leftovers covered in the fridge rather than at room temperature. The pastry will lose its crispness overnight, so warm slices in the oven for a few minutes at around 180°C to bring back some of the flakiness - a microwave will only make them soft. Dust with fresh powdered sugar after reheating, since the first dusting will have dissolved.