Heat over to 350 F. Spray bottom and sides of 9x13 inch baking pan with cooking spray.
In large bowl, beat cake mix, oil, eggs, sour cream, pumpkin and 1 1/2 teaspoons of the pumpkin pie spice with electric mixer on medium speed 2 minutes, scraping bowl occasionally. Spread half of the batter in the pan (about 2 1/2 cups).
In small bowl, mix together brown sugar, pecans and remaining 1 teaspoon pumpkin pie spice; sprinkle evenly over batter in pan. Carefully pour and spread remaining batter over pecan mixture.
Bake 35 to 40 minutes or until deep golden brown and toothpick inserted in center comes out clean. Remove from oven to cooling rack. In another small bowl, stir powdered sugar, milk, and vanilla until thin enough to spread. Carefully spread powdered sugar mixture over warm cake. Cool completely, about 1 hour. Store covered.