Heat oven to 350 degrees. Grease 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, and 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.

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.