Spray 13x9 baking dish with non-stick spray ( you can use the kind with the flour in it)
In a medium bowl, stir together the cake mix, eggs, sour cream, and oil. Mix until well incorporated.
In a separate bowl, mix together the brown sugar and cinnamon until well blended; set aside.
Pour half the cake batter into a greased 13x9 pan, spreading it out evenly. Add all the brown sugar/cinnamon mixture to the top covering the entire layer of the batter. Pour the rest of the cake batter on top and pread the batter out as evenly as possible.
Place the cake in the oven and bake about 40-45 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 of desired consistency. I like mine a bit thick but still pour-able. When the cake comes out of the oven, immediately pour glaze onto the hot cake. Spread it out evenly. Let the cake sit for a few minutes so the glaze can set and serve warm or room temperature.
To serve it warm, pop it in the microwave for about 15 seconds.