Pistachio Pudding Muffins

4 eggs, beaten
1 (18.25 ounce) box Duncan Hines Butter Golden Cake Mix
1 cup all-purpose flour
1 box pistachio instant pudding
1 cup sour cream
1/4 cup water
1/4 cup vegetable oil
1/2 cup chopped pistachio nuts
1 large chocolate bar chopped into chunks (optional)

Sift flour with cake mix. Add instant pudding and mix. Add cake mixture to eggs and stir well. Next, add wet ingredients and stir well. Fold in nuts and/or chocolate chunks. Pour into greased muffin cups. Sprinkle sugar on top. Bake at 425 degrees F for 15 minutes.