Preheat the oven to 350°F (175°C) and prepare a 9" x 13" baking pan by greasing it with butter or non-stick spray and lining with parchment paper.

In a mixing bowl, beat together the softened butter and light brown sugar until smooth and creamy, about 2-3 minutes. Gradually add the all-purpose flour and salt, mixing until crumbly. Press the mixture into the prepared pan, pricking the surface with a fork.

Bake the crust in the preheated oven for 28-32 minutes until lightly golden and firm. Let it cool slightly while preparing the filling.

In a large bowl, whisk together the canned pumpkin, sugar, eggs, and pumpkin pie spice until smooth and incorporated. Pour the filling over the cooled crust.

Return to the oven and bake for an additional 32-35 minutes, until the filling is set but jiggles slightly in the center.

Allow the bars to cool at room temperature for at least an hour before lifting out with parchment edges for slicing.

If making the optional sour cream whip, whisk together the heavy cream, sour cream, sugar, and vanilla until soft peaks form. Serve the bars with the whip or a dusting of powdered sugar.

Store leftover bars in an airtight container in the refrigerator for up to 5 days or freeze for up to 3 months after wrapping tightly.