Preheat oven to 350°F. Line standard muffin pan with 12 paper liners. Whisk both flours in medium bowl. Beat butter in large bowl until smooth. Add sugar; beat to blend. Beat in eggs 1 at a time, then next 3 ingredients (batter may look curdled). Beat in flour mixture in 3 additions alternately with buttermilk in 2 additions. Spoon scant 1/3 cup batter into each liner.
Bake cupcakes until tester inserted into center comes out clean, 20 to 25 minutes. Cool 10 minutes. Remove from pan; cool.
Beat all ingredients in medium bowl until smooth. Spread over cupcakes.