Pinjanpähkinäbiscotit

Yes - chopped almonds, hazelnuts or pistachios work well, since the nuts here sit on top rather than in the dough. Keep the pieces small so they press into the surface and stay put during baking. The 20 g called for is a light topping, so any nut with a similar coarse chop will do.

The dough balls need to be pressed firmly enough that the nuts sink slightly into the surface, not just rest on it. Sprinkle the nuts on first, then press and flatten the ball - the dough closes around them as it spreads. If the butter was too cold, the dough stays stiff and won't hold the nuts.

Yes, the dough keeps well chilled and can be made a day ahead. Let it come back to room temperature before shaping, otherwise the 25 g balls crack when you flatten them. You can also shape and top the cookies, then refrigerate them on the baking sheet until you're ready to bake.

Keep them in an airtight tin at room temperature once they are completely cool - packing them away while still warm traps steam and softens the crisp outside. Don't refrigerate them, as the moisture works against the texture. Add the final dusting of powdered sugar just before serving rather than before storing.

Overbaking is the big one - at 170 °C / 340 °F they're done at 12-15 minutes, when the edges are just golden and the centres still look pale and soft. They firm up as they cool, so pulling them out while they seem slightly underdone is what gives you the tender inside. The second mistake is overmixing after the flour goes in; add it gradually and stop as soon as the dough comes together.