Mandelcroissant-galette

Either you used too much or you spread it too close to the rim. Stick to about 1 heaping teaspoon (roughly 20-25 g) per galette and leave the folded rim bare, since the filling softens and spreads on its own in the oven. Pressing the folded corners down firmly also helps the rim hold its shape and act as a little wall.

Yes - ground hazelnuts or ground pistachios work in the same 70 g quantity and behave the same way in the cream. The texture stays close as long as the nuts are finely ground rather than chopped, since coarse pieces make the filling grainy instead of smooth. Keep the sliced almonds on top or switch them to matching chopped nuts for a consistent look.

Keep them at room temperature in a loosely covered container rather than an airtight one, so the pastry does not go soft from trapped steam. They are best on the day of baking; after that, a few minutes in a 190 °C (375 °F) oven crisps the layers back up. Dust with powdered sugar only after reheating, as it melts away in the heat.

The frangipane can be creamed ahead and kept covered in the fridge - let it soften slightly before spreading, or it will tear the pastry. You can also shape, fill, and top the galettes completely and leave them chilled, which just extends the 10-15 minute chilling step the recipe already calls for. Brush the egg yolk and milk wash on right before they go into the oven for the best shine.

Skipping the chilling step is the biggest one - warm pastry loses its layers and the galettes bake flat instead of puffing at the rim. Using butter that is too cold or too melted also matters, since the frangipane only creams into a smooth mass with genuinely soft butter. Finally, letting the egg wash drip down the cut edges glues the layers together and stops them from rising.