Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch pan. Make the streusel: In a medium bowl, combine the graham cracker crumbs, nuts, brown sugar and cinnamon. Stir in the melted butter. Set aside.
In a large mixing bowl, combine the cake mix, water, oil, and eggs. Beat on low speed with an electric mixer just until moistened. Beat on medium speed for 2 minutes longer. Spread 1/2 of the batter into a greased 9x13 inch pan. Sprinkle on 1/2 of the streusel mixture. Carefully spread the remaining 1/2 of the cake batter over the streusel, and sprinkle with the remaining streusel.
Bake for 35 to 40 minutes or until a toothpick inserted comes out clean. Cool slightly, then drizzle with the powdered sugar icing. Best if served warm. Makes 12 to 16 servings.
To make the Icing: In a small bowl, combine confectioners sugar with vanilla and butter flavoring. Add a few drops of water until you achieve drizzling consistency.