To a large bowl or a food processor, add the crust ingredients (flour, sugar, butter). Cut butter into the dough with a pastry cutter or pulse in a food processor until it has a sandy texture.
Line an 8 x 8 baking dish with parchment paper and press the shortbread crust into place.
To a large bowl, add the pumpkin, sugar, eggs, salt, cinnamon, ginger, cloves, nutmeg, and allspice. Use a whisk to fully mix it together.
Pour in the evaporated milk and use a silicone spatula to fold it in (if you use a whisk for this step, you’ll get too many bubbles which will remain during baking).
Bake the bars at 425 F for 20 minutes, then turn the temperature down to 350 F (176 C) and bake for an additional 45 minutes (or until the bars look mostly set with just a little jiggle and look a bit puffed up).
Remove bars from the oven and allow them to completely cool on a cooling rack. After they’ve reached room temperature, move them to the fridge to allow them to set for a minimum of 2 hours (but preferably overnight).
If you’re making this recipe for a crowd, it doubles beautifully! Just use a 13 x 9 pan.