1. Preheat oven to 350 degrees.Spray a 9x13-inch baking pan with non stick cooking spray and set aside.In a medium bowl blend together the brown sugar and cinnamon; set aside.In a large mixing bowl beat together the cake mix, sour cream, eggs and oil until combined and fluffy.
2. Pour half of the batter into the prepared pan, spread to the edges.Sprinkle on top of the batter all of the brown sugar mixture.Carefully add the remaining batter over the top of the brown sugar mixture.
3. Spread to the edges.
4. Bake 30 to 35 minutes or until edges pull away from the sides and the cake is golden brown.While the cake is baking make the glaze.In a medium bowl add the sugar, milk and vanilla.
5. Whisk until completely combined.As soon as the cake comes out of the oven drizzle glaze over the top.
6. Serve hot or at room temperature.