1) In a food processor, pulse together the pine nuts, flour, cornmeal and salt until finely ground and smooth.
2) Using a handheld or stand mixer, beat together the butter and 65g of icing sugar in a medium bowl until light and creamy. Add the lemon juice, 1 teaspoon of zest and the vanilla, and beat to combine. With the mixer on low speed, add the dry ingredients in three batches until just combined; be careful not to over-mix. Chill the dough in the refrigerator for 30 minutes.
3) In a shallow bowl, combine the remaining sugar and remaining zest. Set aside.
4) Preheat the oven to 180°C/ gas mark 4.
5) Roll the dough into 7-cm balls. Transfer the dough onto baking sheets lined with parchment and bake until firm and very pale golden, about 12 to 15 minutes.
6) While the biscuits are still warm, roll each one in the sugar mixture. To achieve a solid coating, roll each of the cookies a second time in the sugar mixture.