Make the cinnamon layer. In a small bowl, whisk together the brown sugar and cinnamon set to the side.
Make the cake batter. To a large bowl or electric stand mixer, add in the cake mix, eggs, oil, and sour cream, and mix until well combined.
Grease a 9x13 baking dish with non-stick spray and pour half of the batter into the pan and spread out. Sprinkle the brown sugar and cinnamon mixture evenly over the top of the cake batter. Pour the remaining cake batter over the top. Bake for 35 minutes or until cooked through.
While the cake is in the oven, make the glaze. To a medium-sized bowl add the powdered sugar, milk, and vanilla. Whisk until smooth.
Once cake is out of the oven pour the glaze over the top. Serve and enjoy!