Place hazelnuts on baking sheet. Bake until they're a shade darker and fragrant, about 10 to 15 minutes. Remove from oven and set aside to cool.
Beat butter and sugar together with a wooden spoon until smooth. Beat in egg. Gradually beat the flour mixture into the butter mixture. Add the chopped hazelnuts and mix until distributed evenly throughout.
Form into long cookies and place on greased cookie sheets. Bake until firm, about 15 minutes. Remove from oven and cool on racks.