Preheat oven to 350 degrees F. To prepare batter, in a large bowl, beat cake mix, pudding mix, oil, eggs, vanilla, and 1 cup water with an electric mixer on medium speed, 2 minutes. To prepare streusel, in a medium bowl, mix together chocolate chips, walnuts, brown sugar, and cinnamon. Turn half of batter into a greased and floured 12-cup bundt pan. Top evenly with half of streusel mixture. Add remaining batter, then remaining streusel. Swirl streusel gently through batter with a knife. Bake 50-60 minutes, or until a cake tester inserted in center comes out clean. Let cake cool in pan on rack 15-20 minutes, then invert onto a serving plate to cool completely. Before serving, sprinkle with powdered sugar.