Sieve together the hazelnuts and sugars. Add the egg yolk and lemon juice and combine to a marzipan paste. Roll it out very thinly on a surface covered with more icing sugar and use to line the loose-bottomed tartlet tins, trimming off any excess paste. These can now be left out to dry at room temperature for 24 hours. This will allow the marzipan to set and leave very crisp tart cases.