Preheat oven to 350°F. Spray a 9x13 inch baking pan with baking spray (or butter and flour the pan.)
In a large bowl, whisk together the pumpkin puree, evaporated milk, eggs, white and brown sugar, vanilla, and pumpkin pie spice until all ingredients are thoroughly combined and the batter is smooth.
Pour the pumpkin mixture into a prepared pan and spread evenly with a spatula.
In a medium bowl, stir together all the ingredients for the "cake mix" layer until well combined.
Sprinkle the dry "cake mix" over the top of the pumpkin layer.
Sprinkle the top of the cake with chopped walnuts and drizzle the melted butter over the top.
In a small bowl, stir together 1/4 cup sugar with 2 teaspoons cinnamon.
Sprinkle the cinnamon sugar mixture evenly over the top of the cake.
Cover the pan with foil.
Bake in preheated oven for 30 minutes. Remove the foil. Continue baking an additional 25 - 30 minutes or until set.
Remove pan to a wire rack to cool completely.
One serving suggestion is to top with ice cream before serving.