Preheat the oven to 350°F, and grease a 9×13 inch baking pan or line in with parchment paper for easier clean up.
Add the cake mix, melted butter, egg, flour, brown sugar, cinnamon, vanilla, and salt to a large mixing bowl. Stir to combine the ingredients until they form a dough like consistency.
Press two thirds (⅔) of this mixture into the bottom of your prepared pan to form the crust. Set the remaining one third (⅓) of the dough aside to use for the crumble topping. Bake the pie bar crust for 10 to 15 minutes to help it firm up, while you mix the other ingredients.
In a separate large mixing bowl, whisk together the pumpkin puree, evaporated milk, eggs, melted butter (2 Tbsp), granulated sugar, brown sugar, spices, salt, and vanilla extract until the mixture is smooth and the ingredients are well combined.
Pour the pumpkin mixture over the baked pie bar crust in the pan. Take the remaining one third (⅓) of the cake mix mixture and crumble it evenly over the pumpkin filling. You can add a bit more cinnamon or even some chopped nuts or chocolate chips to the crumble if for additional flavor and texture.
Bake the pumpkin pie bars for 40 to 45 minutes, or until the pumpkin filling is set and the topping is golden brown.
Let the pumpkin pie bars cool completely before slicing. Serve the bars with whipped cream, vanilla ice cream, or caramel sauce for a delicious dessert.