Combine. In a large mixing bowl, combine pumpkin puree, brown sugar, eggs, cornstarch, vanilla, spices, and salt. Whisk until this mixture is smooth with NO lumps. You don't want any small pieces of egg white, brown sugar, or cornstarch. It should be as uniform as possible.
Gently Whisk In Evaporated Milk. This will lighten the color significantly. Again, you want the mixture smooth and uniform, but use a gentle whisking motion to avoid adding air bubbles to the pie filling.
Pour pie filling over the slightly cooled crust (it's ok if it's a little warm, you just don't want it piping hot from the oven!). Smooth into an even layer as needed. Place the 8x8 pan on a sheet pan.
Bake 45-55 minutes, or until slightly wobbly in the center but set around the sides. The residual heat will continue to set the pie filling as it cools, so a slight wobble in the center (but no liquid sloshing!) is what you're going for.
Cool the pie bars 30-60 minutes at room temperature, then chill at least 4 hours to cool completely. (This is important, as it also helps the filling set up!)
Serve & Store. When the bars are fully chilled, lift the bars out of the pan with the parchment paper and cut into slices. You can do 9 very large squares, 12 rectangles, or 16 small squares (a great option on Thanksgiving!) Serve individual slices with whipped cream or ice cream and extra garnishes, like pecans or caramel sauce, as desired. Store leftover pumpkin pie bars in an airtight container (or covered in the pan) in the fridge 3-4 days. Or freeze up to 2 months!