1.Preheat oven to 160°C. Line oven trays with baking paper.
2.In a small bowl, beat butter, extract and sugar with an electric mixer until light and fluffy. Beat in egg and stir in ground hazelnuts and sifted flour.
3.Flour hands and roll level tbsp of mixture into balls. Place about 5cm apart on trays and flatten slightly. Using the end of a wooden spoon, press a hole into the centre of each biscuit. Fill each hole with chocolate-hazelnut spread.
4.Bake biscuits about 20 minutes. Cool on trays. Dust biscuits with sifted cocoa before serving.