1.Preheat oven to 180°C/350°F. Grease oven trays; line with baking paper.
2.Beat butter and sugar in a medium bowl with an electric mixer until smooth. Stir in sifted ground hazelnuts and flour to form a stiff dough. Roll level teaspoons of mixture into balls, place on an oven tray; flatten into 3.5cm (1½-inch) rounds. Using a finger, smooth out any cracks around the edges (this may take some time as the dough is quite stiff ).
3.Bake biscuits for 8 minutes or until browned lightly. Stand for 5 minutes before transferring to a wire rack to cool.
4.Meanwhile, make chocolate filling. Place chocolate and butter in a small heatproof bowl over a small saucepan of simmering water (don’t let water touch base of bowl); stir until smooth. Stand at room temperature until thickened to a spreadable consistency; stir in chopped hazelnuts.
5.Sandwich cooled biscuits with 2 teaspoons of chocolate filling.
Plain biscuits will keep in an airtight container for up to 1 week. Sandwiched biscuits will keep, refrigerated, in an airtight container for up to 1 week. These bite-size biscuits hail from Piedmont in Italy, a region famed for the combination of chocolate and hazelnuts. Ground nuts lose their flavour quickly so, for a more intense hazelnut flavour, finely process 200g (6½ ounces) whole hazelnuts. Sift out large pieces, and weigh the amount required.