Step 1

Preheat oven to 325 degrees and spray a 9x13-inch baking dish with cooking spray.

Step 2

In a large bowl, stir together cake mix, eggs, vegetable oil, and sour cream until smooth.

Step 3

Pour half of cake batter into prepared pan and smooth it out.

Step 4

Stir together brown sugar and cinnamon in a bowl. Sprinkle evenly on top of cake batter.

Step 5

Cover with remaining cake batter, spreading it evenly. Run a knife in a swirling pattern through the cake batter.

Step 6

Bake 30 to 35 minutes.

Step 7

In a medium bowl, whisk together powdered sugar, milk, and vanilla.  Pour over hot cake. Let cake cool 20 minutes before cutting.