Preheat the oven to 170°C/375°F/Gas Mark 5. 

In a mixing bowl, beat together the butter and sugar until light and creamy. Beat in the egg yolk and stir in the flour, salt, Baking Powder, Natural Vanilla Extract, Chocolate Chunks and hazelnuts to make a firm dough. 

Drop tablespoons of the mixture, well spaced apart, on baking trays lined with greaseproof paper. Bake in the oven for about 10 minutes until firm and lightly browned. Cool for 10 minutes on the baking trays, then transfer them to wire racks to cool further. Best served slightly warm.