Mix the cake mix, cornstarch, powdered sugar, eggs, sour cream, and oil together until smooth in a large bowl. Pour the batter into 2 floured and greased 8x4-inch loaf pans. If desired, combine the brown sugar and cinnamon. Spoon the cinnamon mixture over the batter in the pan and swirl with a knife to mix together. Bake at 350 degrees for 40 to 45 minutes or until toothpick inserted in center comes out clean. Cool for 10 minutes. Loosen the edges of the cakes with a knife; turn onto a cooling rack. Cool completely before serving.