1 (18.25 ounce) package yellow cake mix
1 (3.5 ounce) package instant vanilla pudding mix
1 cup milk
3/4 cup vegetable oil
1 teaspoon butter
1 teaspoon vanilla extract
1 cup brown sugar
1 cup chopped walnuts
2 teaspoons ground cinnamon
Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch casserole dish.
Mix 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.
Stir brown sugar, walnuts, and cinnamon together in a bowl; sprinkle over batter in the casserole dish. Pour remaining batter over walnut mixture.
Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, about 40 minutes.