Preheat oven to 325 degrees F. Spray a 9 x 13-inch pan with cooking spray and set aside.
In a large bowl, combine cake mix, eggs, oil and sour cream. Using an electric mixer, beat for 3 minutes on medium speed. Pour half of the batter into the prepared pan.
In a small bowl, stir together brown sugar and cinnamon. Sprinkle over the batter in the pan.
Spread remaining batter over top. Bake for 44-48 minutes, or until golden brown and a toothpick inserted in the center of the cake comes out clean.
Allow the cake to cool for about 5-10 minutes. Use a fork to prick holes all over the top of the warm cake – especially in the corners.
While cake is still warm, whisk together powdered sugar, vanilla and milk until smooth. You want it to be thin enough to pour and spread over the top of the cake. If it seems too thin, add some extra powdered sugar. If it seems too thick, add some extra milk. Pour evenly over the top of the warm cake. Slice and serve warm, or allow to cool to room temperature.