In a large heavy-duty stand mixer, combine the flour, brown sugar, cinnamon, vanilla and butter. Beat until butter is well incorporated and resembles a stiff cookie dough. Remove from bowl and set aside.

Preheat oven to 350 degrees. Spray or grease a 9-inch square pan. Prepare cake mix as directed on the box. Pour 1/2 the batter into the prepared pan. Sprinkle the batter with the cinnamon sugar if using; maybe about 1/4 cup well distributed across the cake. Top with remaining batter and swirl lightly.

Bake cake for 10-15 minutes or until cake is just beginning to rise and is set in the center. Remove pan from oven and top with crumbs by breaking off marble-sized portions of the crumb mixture and distributing evenly across the top of the cake. The top of the cake should not show through the crumbs and the crumb layer will be THICK, and the center of the cake will fall slightly. This is OK.

Return pan to the oven and bake for an additional 15-20 minutes, or until crumbs lose their wet look and are beginning to brown.

Remove from oven and cool completely on rack. Dust with confectioner's sugar, if desired, when completely cool. Makes 9 servings.