Line an 8×8 or 9×9 inch square pan with parchment paper. Let the parchment overhang on at least two sides so you can lift out the bars easily later.
In a medium bowl, stir together the graham cracker crumbs, brown sugar and cinnamon. Pour in the melted butter. Mix until all the crumbs are evenly moistened and look like wet sand.
Press the mixture firmly into the bottom of the lined pan. Use the bottom of a measuring cup or a flat glass to press it down tightly and evenly. Refrigerate while you make the filling; for a firmer crust, pop it in the freezer for 15 to 20 minutes.
In a large mixing bowl, beat the softened cream cheese with a hand mixer or stand mixer on medium speed until it is completely smooth and fluffy with no lumps.
Add the powdered sugar to the cream cheese. Beat again until smooth and creamy, scraping down the sides and bottom of the bowl as needed.
Add the pumpkin purée, vanilla extract, pumpkin pie spice and a pinch of salt. Mix on low speed just until everything is smooth and evenly combined. Do not overmix; you want it creamy but not runny.
Switch to a rubber spatula. Gently fold in the thawed Cool Whip until no white streaks remain. Use slow, sweeping motions from the bottom of the bowl up to keep the filling light and airy.
Pour the pumpkin cheesecake filling over the chilled graham cracker crust. Spread it out evenly and smooth the top with an offset spatula or the back of a spoon.
Chill until set. Loosely cover the pan with plastic wrap or foil, making sure it does not touch the surface of the filling. Refrigerate for at least 6 hours or overnight until the no bake pumpkin cheesecake is fully set and firm enough to slice.
Use the parchment overhang to lift the cheesecake slab out of the pan and onto a cutting board. Use a sharp knife to cut into 16 bars, wiping the knife clean between cuts for neat edges.
Add your favorite toppings and serve chilled.