In a large mixing bowl, mix cake mix, sour cream, eggs and oil by hand until large lumps are gone. Pour half of the batter into the pan.
In a small bowl, mix brown sugar and cinnamon and then sprinkle over the top of the batter in the pan.
Pour the remaining batter over the cinnamon layer and spread out evenly. It's a little tough to do and it may get a bit messy, but just do your best. It will be fine. :)
Drag a butter knife through the batter to swirl the cinnamon and cake batter.
Bake for 35-40 minutes, or until a toothpick comes out clean.
During the last few minutes of baking, mix the powdered sugar, milk and vanilla in a bowl to create the glaze. Use a fork to mix it up well and break up any lumps. Pour directly over the hot cake as soon as you pull it from the oven.
Let sit for a few minutes and then serve warm with a cup of coffee or a tall glass of milk.