1Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch casserole dish.
2Mix yellow cake mix, vanilla pudding mix, milk, vegetable oil, eggs, butter, and vanilla extract together in a bowl. Pour half the batter into the prepared casserole dish; spread evenly.
3Stir brown sugar, walnuts, and cinnamon together in a bowl; sprinkle over batter in the casserole dish. Pour remaining batter over walnut mixture.
4Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, about 40 minutes.
Footnotes
Tip
Aluminum foil helps keep food moist, ensures it cooks evenly, keeps leftovers fresh, and makes clean-up easy.