Heat oven to 300 degrees. Beat butter until it is pale. Gradually add sugar and beat until smooth, 5-7 minutes. Stir in flour. Briefly knead batter until soft dough forms. Roll into small balls; place 1 inch apart on ungreased baking sheets. Flatten each ball on one side so that half of the cookie is flattened and the other is not. Press a hazelnut into middle of the higher side. Bake 20 minutes. Cool completely before serving.
