Preheat oven to 350°F. and butter and flour a 10 x 4-inch (4-quart) tube pan.
Coarsely chop walnuts. In a large bowl with an electric mixer cream butter with sugar until light and fluffy. On low speed add flour, salt, and walnuts, beating until combined well, and beat in lemon extract and vanilla. Add eggs 1 at a time, beating well after each addition, and beat batter on high speed 5 minutes.
Pour batter into tube pan and bake in middle of oven 1 1/4 hours. Cool cake in pan on a rack 30 minutes. Turn cake out onto rack and cool completely.