Step 1

Preheat oven to 350 degrees. In a large bowl, combine cake mix, water, buttermilk, oil, eggs, and pudding mix. Beat with a mixer for about 2 minutes. Fold in sour cream.

Step 2

In a smaller bowl, combine sugars, flour, cinnamon, and salt. Cut butter into small pieces and add to the sugar mixture. Mix together with a fork or pastry blender until crumbly. Stir in walnuts.

Step 3

Pour batter into a greased 9x13 glass baking dish. Sprinkle half of the topping over the batter. Use a knife to swirl the mixture throughout the cake.

Step 4

Top evenly with the remaining topping.

Step 5

Bake for 35 to 45 minutes or until a toothpick inserted into the center comes out clean.