Combine cake mix, eggs, sour cream, and oil (mixture may be slightly lumpy). Set aside.
In a separate bowl, mix together the brown sugar and cinnamon, set aside.
Pour half of the cake batter into a greased 9 x 13 inch baking dish, spreading the batter out evenly. Add half of the swirl mixture to the top, covering the entire layer of batter. Pour the remaining batter on top, and spread out until even. Top with the rest of the swirl mixture.
With a knife, swirl the batter together in a circular motion. Bake for 45-50 minutes, or until cake tests done with a toothpick.
To make the glaze, mix together the powdered sugar, milk, and vanilla until glaze is smooth and lump free.
Top cake immediately with glaze as soon as it comes out of the oven.
Let cool before serving.
You can easily make this gluten-free by using a GF cake mix.