Heat oven to 350°F (325°F for dark or nonstick pan).

Spray bottom and sides of 13×9-inch pan with cooking spray.

In large bowl, beat cake mix, oil, eggs, and sour cream with electric mixer on medium speed 2 minutes, scraping bowl occasionally.

Spread half of the batter in pan.

In small bowl, stir together brown sugar, pecans and cinnamon;

Sprinkle evenly over batter in pan.

Carefully pour and spread remaining batter evenly over pecan mixture.

Bake 28 to 32 minutes or until golden brown and toothpick inserted in center comes out clean.

Remove from oven to cooling rack.

In another small bowl, stir together powdered sugar, milk, and vanilla until thin enough to spread. If powdered sugar mixture is too stiff to spread, add additional milk 1/4 teaspoon at a time.

Poke entire surface of warm cake with a fork.

Carefully pour and spread powdered sugar mixture over warm cake.

Cool completely, about 2 hours. Store covered.