Preheat oven to 350 degrees. Well butter a bundt pan. Sprinkle with 1/2 cup finely chopped walnuts, then sprinkle with flour. Set aside.

In small bowl, combine 1/2 cup (packed) brown sugar and 1/2 cup white sugar and 2 level teaspoons cinnamon; mix well and set aside.

In large mixing bowl combine:

1 pkg. white or yellow cake mix
1 pkg. instant vanilla pudding mix
1 c. water
1 tbsp. soft butter
4 eggs

Beat well with electric mixer for approximately 5 minutes.

Carefully spoon 1/2 the batter into the bundt pan. Sprinkle with 1/2 the sugar cinnamon mixture. Spoon the remaining batter carefully into the pan and sprinkle with the remaining sugar mixture. Carefully swirl the sugar mixture into the batter.

Bake for 50 to 60 minutes. Let stand in the pan for 15 to 20 minutes, then remove. Cool and sift powdered sugar over the top.