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The dough is meant to be soft, but if it sticks to your hands it usually means the butter was too warm or the flour wasn't measured accurately. Chill the dough for 20-30 minutes in the fridge, then shape the walnut-sized balls with cool hands. Since there is only one egg yolk and no liquid, the dough firms up reliably once the butter is cold again.

Yes - ground almonds work as a one-to-one swap for the 100 g of ground hazelnuts and give a milder, slightly sweeter cookie. Ground walnuts or pecans also work but taste more assertive. Keep the flour, baking powder and salt exactly as written, because the nuts here carry structure as well as flavour.

Go by look, not by clock: after 13-15 minutes the surfaces should show fine cracks while the cookies are still pale, with only the bottom edges lightly golden. They will feel very soft in the oven - that is normal, because they firm up as they cool. If you wait for them to look browned all over, they will be dry and lose their crumbly texture.

Yes, the dough keeps well wrapped in the fridge for up to two days; let it sit at room temperature for a few minutes before shaping so it isn't crumbly. You can also shape the balls, press in the whole hazelnuts, and freeze them on a tray, then bake from frozen with a couple of extra minutes. Do the optional powdered-sugar rolling right before baking, not before storing.

Let them cool completely first - warm cookies release steam and go soft in a sealed box. Stored in an airtight tin at room temperature they stay crumbly and nutty for about a week. If you rolled them in powdered sugar, dust them again lightly before serving, since the sugar tends to be absorbed during storage.