Line a couple of baking sheets with baking parchment.
Grind the roasted skinned hazelnuts to a fine flour in a food processor, blender or spice grinder. Be careful not to go too far or they will become oily and start turning into nut butter.
Mix together the butter, sugar, vanilla and salt with a wooden spoon or spatula until well combined. Don't beat the mixture for too long however, you don't want to incorporate too much air.
Add the ground hazelnuts, plain flour and cornstarch. Mix with a wooden spoon or spatula until the dough starts coming together, then gently knead it by hand until smooth.
Roll the dough out on a lightly floured surface to about 8mm (⅓ in) thick. Use your choice of cutters to cut out as many cookies as possible. Bring together the scraps and re-roll to cut out more cookies. (If the dough is too soft to roll out, wrap and chill it in the fridge for an hour first).
Place the cookies slightly spaced apart on the baking sheets. Pop them in the freezer for 15 minutes while you preheat the oven to 160℃/140℃ fan/325℉/gas mark 3.
Bake the chilled cookies for 25-30 minutes until pale golden. Allow to cool on the baking sheets for a couple of minutes then transfer to a wire rack and allow to cool completely.
Once the cookies have cooled, melt the chocolate, either in a bain marie or in short bursts in the microwave. Transfer it to a deep but narrow container, such as a wide mug. Check that the biscuits will fit in it first though!
Dunk one half of the cookies in the chocolate, shake off the excess then place them on a wire rack. Sprinkle with chopped hazelnuts before the chocolate sets.