Preheat oven to 325 degrees.
Spray 9 x 13 baking dish with nonstick spray.
In a medium bowl stir together the cake mix, eggs, sour cream, and oil and mix until well-incorporated.
In a separate bowl, mix together the brown sugar and cinnamon until well-blended; set aside.
Pour half of the cake batter into a greased 13×9 pan, spreading it out evenly.
Add all of the brown sugar/cinnamon mixture to the top, covering the entire layer of batter.
Pour the rest of the cake batter on top, and spread the batter out as evenly as possible.
Place the cake into the oven and bake for about 40 minutes, or until an inserted toothpick comes out clean.
While the cake is baking, make the glaze.
In a bowl, mix the powdered sugar, milk, and vanilla until glaze is smooth.
When the cake comes out of the oven, immediately pour glaze onto the hot cake.
Spread it out evenly.