Preheat oven to 180ºC Toast hazelnuts, husk and coarsely chop; set aside. Lightly butter 9x13 inch baking pan. Melt 1 cup butter in heavy large saucepan over low heat. Remove from heat and whisk in sugar, eggs, cocoa and vanilla. Stir in flour and salt. Add chopped chocolate and nuts. Spread batter in prepared pan. Bake until tester inserted in center comes out barely moist but not wet, 25 to 30 minutes. Cool in pan on rack. (Can be prepared 1 day ahead. Wrap tightly.) Cut into 4 cm squares.