Preheat oven to 350 degrees F. Coat a 9 x 13-inch baking dish with cooking spray.

In a medium bowl, combine graham cracker crumbs, brown sugar, walnuts, cinnamon, and butter; mix well and set aside.

In a large bowl, prepare cake mix. Pour half the batter into baking dish and sprinkle with half the crumb mixture. Repeat with remaining batter and crumb mixture.

Bake 25 to 30 minutes, or until a toothpick inserted in center comes out clean. Let cool, cut into squares, and serve.