Thread the langoustines carefully onto the skewers and chill for up to a day. For the butter, finely grate or chop half the hazelnuts (grating gives the nuttiest flavour), then roughly chop the rest. Tip into a small saucepan with the butter, parsley and a little seasoning, then set aside.
When ready to serve, melt the hazelnut butter in the pan. Heat a griddle pan or barbecue, then brush the langoustines with the oil. Griddle each skewer for 1-2 mins, turning, until pink and just cooked. Pile on a platter, pour half the hazelnut butter over the langoustines, then serve the rest in a small bowl for dipping.