Preheat oven to 350 degrees F (or 325 degrees F for dark or non-stick pan). Spray 9 x 13" pan with baking spray with flour. Set aside.

In a small bowl combine the pecans, cinnamon, and brown sugar. Set aside. In a large bowl, beat cake mix, oil, eggs, and sour cream with a mixer on low speed for 30 seconds then medium speed for 2 minutes, occasionally scraping downsides. Spread half the batter into the prepared pan. Sprinkle pecan mixture over batter. Carefully spread the remaining batter over the pecan mixture.

Bake for 45 to 50 minutes or until deep golden brown. Remove from oven and cool for 5 minutes. 

In another small bowl, combine powdered sugar, milk, and vanilla. Spread mixture over the cake while warm. Cool completely. Store covered.