In a mixing bowl, combine all-purpose flour and both granulated and brown sugar, blending well. Cut in unsalted butter using a pastry cutter until the mixture resembles coarse crumbs. Add a pinch of salt and a splash of cold water, mixing until a dough forms. Press this mixture evenly into a lined 9x13-inch baking pan and pre-bake in a preheated oven at 350°F for 10 minutes until lightly golden.

While the crust bakes, prepare the filling. In a large bowl, whisk together pumpkin puree, the two sugars, and your chosen spices until smooth. Beat in the eggs, ensuring everything is well combined. Slowly pour in evaporated milk and a dash of vanilla extract, mixing continuously until creamy.

Once your crust has pre-baked, carefully pour the pumpkin filling over the warm crust and return the pan to the oven. Bake for 40-45 minutes. The edges should be firm while the center might still jiggle slightly when done.

After baking, remove the pan from the oven and allow to cool completely, at least 2 hours. Once cool, lift the bars out using the parchment paper and cut into squares for serving.