Preheat oven to 325 degrees. By hand mix together cake mix, eggs, sour cream, and oil until well combined. Set aside. In a seperate bowl combine brown sugar and cinnamon and mix together until well combined. Pour half of cake batter in a greased 13×9 pan. Spread it out good.
Add half of brown sugar mix on top of that covering all the batter. Pour rest of batter and spread until even. Top with rest of brown sugar mix. With a knife swirl your batter around. Bake 45-50 minutes until a toothpick comes out clean. Meanwhile mix ingredients for frosting. When cake comes out of the oven pour the frosting on the hot cake. Let cool or serve warm.