In the bowl of an electric mixer, combine butter and sugar; beat until thoroughly blended. Add the rest of the ingredients except walnuts to the electric mixer bowl and beat at low until dry ingredients are moistened. Turn to high and beat until smooth. Blend in walnuts by hand. Spoon batter into muffin cups until 2/3 full.

Reduce oven temperature to 350°F and bake for 15 to 17 minutes or until a toothpick inserted in the center comes out clean and dry. Remove from oven and allow to cool in pan for 10 minutes before turning out.