In a mixing bowl, whisk together the cake mix, eggs, sour cream, vegetable and oil to make a smooth batter. In a separate bowl, stir together the brown sugar with cinnamon. Pour half the cake batter into the prepared baking pan, and sprinkle the brown sugar-cinnamon mixture over the batter. Pour the remaining batter over the cinnamon mixture, and gently swirl the batter a few times with a table knife.

Bake the cake in the preheated oven until lightly golden brown and a toothpick inserted into the center of the cake comes out clean, about 40 minutes.

Cheat: use canned frosting vs. homemade and spread while cake is hot. only about 1/2 tub.