Preheat the oven to 350°F. Spray a 9x13-inch baking dish with nonstick cooking spray and set aside.

In a large mixing bowl, combine the cake mix, eggs, vegetable oil, and sour cream. Mix with a hand mixer until smooth and fully combined. Scrape down the sides of the bowl to make sure no dry cake mix remains.

Pour half of the batter into the prepared baking dish and spread evenly.

In a small mixing bowl, stir together the brown sugar and cinnamon. Sprinkle the mixture evenly over the batter.

Pour the remaining batter over the cinnamon sugar layer and gently spread it out with a knife or spoon.

Bake for 30 to 35 minutes, or until the center is set and a toothpick inserted in the middle comes out clean.

Let the cake cool completely for 30 to 45 minutes.

To make the glaze, combine the powdered sugar, vanilla extract, and milk in a medium bowl. Stir until smooth.

If the glaze is too thick, add a little more milk.

If it is too thin, add a little more powdered sugar.

Pour the glaze over the cooled cake and spread evenly.

Let the glaze set for about 25 minutes before slicing and serving. You can refrigerate the cake to speed this up.