Banana sour cream cupcakes

1 pkg yellow cake mix
3 eggs
1 c mashed ripe bananas (about 3)
1 c sour cream
1/4 c oil
1 8 oz package cream cheese, softened
1 stick (1/2 c) butter, softened
1 pkg (16 oz or about 4 c) powdered sugar
1 c walnut peices, finely chopped

Directions:
heat oven to 350. Beat cake mix, eggs, bananas, sour cream and oil with mixer on low until moistened. beat on medium 2 minutes.
Bake 18-22 minutes or until toothpick comes out clean.
beat cream cheese and butter with mixer on medium until blended. gradually add sugar, beat well after each addition.
frost cupcakes, then top with walnut peices.