Preheat the oven to 160ºC/gas 2-3 and line two baking trays with greaseproof paper.
Place the nuts on one of the trays and toast in the oven for 10 minutes, or until lightly golden.
Cube the butter and place in a free-standing mixer and beat until pale and creamy. Sift in the icing sugar, then add the brandy and egg yolk, and continue to mix for a few more minutes.
Finely chop the toasted nuts or blitz in a food processor, then add them to the butter mixture, along with the flour, baking powder and pinch of sea salt, then, using a metal spoon, fold it all together, until combined.
Roll spoonfuls of the mixture into balls, then mould each one into a traditional crescent shape – you could also make them into fingers, or cut them with a star cutter, whatever you like.
Place the biscuits on the second lined baking tray and bake for 15 to 20 minutes, or until lightly golden.
Add a generous amount of icing sugar to a small, deep tray. Very lightly brush the warm biscuits with orange blossom water, then pop them in the icing sugar tray and toss to coat.
Transfer to a wire rack and leave to cool completely.