Prep the pan and oven: Heat the oven to 350°F (175°C).
Line a 9x13-inch baking pan with parchment paper, leaving overhang for easy lifting. Lightly grease the sides.
Make the crust: In a bowl, whisk flour, powdered sugar, and salt. Stir in melted butter and vanilla until the dough resembles damp sand and clumps when pressed.
Press and par-bake: Press the crust mixture firmly and evenly into the pan.
Use the bottom of a measuring cup to level it out. Bake for 15 minutes until set and just starting to color.
Mix the filling: In a large bowl, whisk pumpkin purée, brown sugar, and granulated sugar until smooth. Whisk in eggs, then evaporated milk, spices, salt, and vanilla.
The batter should be silky and pourable.
Pour and bake: Carefully pour the filling over the warm crust. Tap the pan gently to pop any air bubbles. Bake 30–40 minutes, until the center is set but still has a slight wobble when you jiggle the pan.
Cool completely: Place the pan on a wire rack and let the bars cool to room temperature.
Then refrigerate for at least 2 hours to fully set.
Slice and serve: Use the parchment overhang to lift the slab out. Cut into 16–24 squares. Top with whipped cream and a pinch of cinnamon if you like.