Preheat the oven to 425°F.
Add the softened butter to a medium mixing bowl and beat on medium speed for 30 seconds. Add the powdered sugar and mix well, then add the flour and mix on low speed until incorporated. Mixture will be soft and crumbly.
Press mixture onto the bottom of a greased 9x13 inch pan.
In a large mixing bowl, add the pumpkin puree, brown sugar, eggs, pumpkin pie spice and salt. Mix on medium speed until combined. Add the evaporated milk and mix until incorporated.
Pour the filling over the prepared crust. Bake at 425°F for 15 minutes, then lower the heat to 350°F and bake 50-55 minutes, until filling is just slightly jiggly in the center. Remove from oven.
Chill in the refrigerator for 2 hours to set. Cut into squares and top with whipped cream as desired.