Coffee Cake

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.