Preheat oven to 350°F. Grease a 13x9-inch baking dish.
In a large bowl, stir together the cake mix, oil, eggs, and sour cream until smooth.
Pour half the batter into the greased baking dish.
In a small bowl, mix together the brown sugar and cinnamon. Sprinkle over the batter in the baking dish.
Top with the remaining batter. Use a knife to lightly swirl the cinnamon into the cake batter.
Bake for 35 minutes, or until golden brown on the edges.
When the cake gets out of the oven, whisk together the powdered sugar, milk, and vanilla extract. Pour over the warm cake.
Let cool, slice into pieces, and serve.
