Heat oven to 350 degrees. Spray 9x13-inch baking pan with cooking spray.
In a 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 for 2 minutes, scraping bowl occasionally.
Spread half of the batter in the prepared pan (about 2 1/4 cups).
In a small bowl, stir together brown sugar, pecans and remaining 1 teaspoon pumpkin pie spice; sprinkle evenly over batter in pan. Carefully pour and spread remaining batter evenly over brown sugar mixture.
Bake in preheated oven for 38 to 42 minutes or until the cake is a deep golden brown color and/or a toothpick inserted in the center of the cake comes out clean. Remove cake from oven.
In another small bowl, stir powdered sugar, milk and vanilla until thin enough to spread over cake. Carefully spread powdered sugar mixture over warm cake. Cool completely (takes about 1 hour). Store covered.