Preheat oven to 350 degrees F. Butter and flour inside of tube pan. In a large mixing bowl, cream butter and sugar until fluffy. Add egg yolks one at a time until mixture is combined; set aside. In a large bowl, stir together flour, walnuts, baking powder, mace and salt; set aside.
Add vanilla and milk to butter mixture; beat until smooth. Add half the flour mixture, beating just until combined. Add remaining flour mixture.
In a small bowl, beat egg whites until soft peaks form. Fold egg whites into cake batter. Pour batter into prepared tube pan. Bake for approximately 1 hour and 15 to 20 minutes, or until top is light brown. Cool before removing from pan. Makes 12 servings.