Preheat oven to 350°F. Spray a 13x9-inch baking dish with baking spray.
In a large bowl whisk together pumpkin puree, evaporated milk, vanilla extract, granulated sugar, brown sugar, flour, pumpkin pie spice, salt and eggs. Whip until fully combined then pour the pumpkin mixture into the prepared baking dish.
Sprinkle dry cake mix evenly all over the top covering the pumpkin filling completely.
Sprinkle the top with pecans and toffee bits. Pour melted butter evenly over pecans and toffee bits.
Bake for 50-60 minutes or until the center is et when gently shaken and the top is golden brown. (Check at 40 minutes and lay a piece of aluminum foil loosely on top to prevent over browning, if needed.)
Cool in the pan for 2 hours on a wire rack, then chill for at least 2 hours before cutting. Store leftovers chilled.
Fresh Whipped Cream:Chill a large mixing bowl and beaters in the freezer for 10-15 minutes. Next, using an electric mixer beat 2 cups of heavy cream with one teaspoon of vanilla extract. Whip on medium speed, gradually adding 1/4-1/3 cup of granulated sugar beating until stiff peaks form. Don't overbeat. This will yield around 4 cups of whipped cream. Store whipped cream chilled until serving. Homemade whipped cream is best used the same day it's made.
Notes
DryCake Mix - You can use your favorite brand of cake mix for this recipe. A yellow cake mix, butter cake mix, butter pecan cake mix or a spice cake mix could be used in this recipe. When using a spice cake mix, you will need to adapt the amount of pumpkin pie spice or omit it altogether.
Cold Butter Versus Melted Butter - Traditional dump cakes use pats of cold butter on top of the cake. While that technique does work for most dump cakes, for this pumpkin variation I find that melted butter works best.
Pumpkin Pie Spice - You can adapt the pumpkin pie spice using all cinnamon or make your own. For this recipe use 1 teaspoon ground cinnamon (in addition to the cinnamon called for in the recipe), 1/4 teaspoon ginger, 1/4 teaspoon, nutmeg, 1/4 teaspoon allspice, 1/4 teaspoon ground cloves and a dash of cardamom.
Nuts - You can use pecans or walnuts for the topping.
Whipped Cream - You can use thawed whipped topping such as Cool Whip or homemade sweetened whipped cream for the topping. See the cook's notes for how to make your own.