HEAT oven to 400°F. Line 16 muffin cups with foil or paper baking cups. Combine all cupcake ingredients except walnuts in large bowl; stir 50 to 75 strokes with spoon until mix is moistened. Stir in 1/2 cup walnuts. Spoon batter evenly into prepared muffin cups.
BAKE 16 to 20 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes. Remove from pan. Cool 30 minutes or until completely cooled.
COMBINE all frosting ingredients except powdered sugar and walnuts in small bowl; beat at low speed until smooth. Gradually add powdered sugar, beating until mixture is of spreading consistency. Frost cooled cupcakes. Immediately sprinkle with 1/4 cup walnuts. Store in refrigerator.
TIP: To substitute for buttermilk, use 1 tablespoon vinegar or lemon juice plus milk to make 1 cup.
High Altitude (above 3500 ft.):
LINE 17 muffin cups with foil or paper baking cups. Add 2 tablespoons flour to dry bread mix. Bake as directed above.