Heat oven to 350 degrees. Greased the bottom and sides of a 13x9 inch baking pan.Place the cake mix, oil,eggs, sour cream, pumpkin and pumpkin pie spice in a large bowl. Beat with an electric mixer on medium speed about 2 minutes until ingredients are just incorporated.  Measure out 2 cups of batter, place the batter onto the bottom of your prepared pan.In a small bowl combine the walnuts with the brown sugar. Toss to combine. Sprinkle evenly over the cake batter.  Spoon the remaining batter evenly over the filling. Using a spatula, carefully spread batter over the filling to the edges of the pan.  Place in the oven and bake about 35 - 40 minutes or until a toothpick inserted comes out clean. Remove the pan to a cooling rack.Make the glaze:Place the powdered sugar, milk and vanilla in a medium bowl. Whisk to combine. Pour the glaze over the top of the hot cake, using a spatula to cover the entire cake with glaze. Cool the cake completely before serving.