Preheat oven to 325°F Grease a 9 x 13 inch baking pan.
Mix together cake mix, eggs, oil and sour cream. Pour into prepared baking pan.
In a separate bowl combine brown sugar and cinnamon. Sprinkle evenly over cake batter then swirl into batter with a knife. The more you swirl, the more combined it will be so don't over-swirl otherwise the cinnamon/sugar will not be swirly.
Bake for 40 minutes.
For icing (optional) combine milk with icing sugar and mix thoroughly. Pour over slightly cooled (10-15 minutes on cooling rack) cake.