Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
In a large mixing bowl, combine the cake mix, eggs, sour cream, and vegetable oil. Mix until well blended.
Pour half of the batter into the prepared baking dish.
In a small bowl, mix together the brown sugar and cinnamon. Sprinkle this mixture over the batter in the baking dish.
Pour the remaining batter over the cinnamon-sugar mixture in the baking dish.
Use a knife to swirl the batter and cinnamon-sugar mixture together.
Bake in the preheated oven for 40 minutes, or until a toothpick inserted into the center comes out clean.
While the cake is baking, prepare the glaze. In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth.
When the cake is done baking, remove it from the oven and let it cool for about 10 minutes. Drizzle the glaze over the warm cake.
Let the cake cool completely before serving. Enjoy!