Preheat the oven to 325°F (163°C). Lightly coat a 9-inch square baking pan with cooking spray. Line the pan with foil, allowing several inches to extend beyond the edges so the chilled bars can be lifted out easily. Lightly spray the foil with additional cooking spray, then set the prepared pan aside.
Place the graham cracker crumbs, melted unsalted butter, granulated sugar, and salt in a mixing bowl. Stir thoroughly until all of the crumbs are evenly coated and moistened.
Transfer the crumb mixture to the prepared baking pan. Press it firmly and evenly across the bottom to form a uniform crust.
Bake the crust at 325°F (163°C) for 5 minutes. Remove the pan from the oven and place it on a wire rack to cool while the pumpkin filling is prepared.
Increase the oven temperature to 375°F (190°C).
Combine the room-temperature eggs and granulated sugar in a large mixing bowl. Using an electric mixer on medium speed, beat for approximately 1–2 minutes, until the mixture becomes pale and noticeably thickened.
Add the vanilla extract, cinnamon, ginger, nutmeg, cloves, and salt. Beat until the spices and flavorings are evenly distributed.
Add the pure pumpkin purée and heavy cream. Continue beating until the filling is completely blended and smooth.
Pour the prepared pumpkin mixture evenly over the cooled graham cracker crust.
Bake at 375°F (190°C) for 32–40 minutes. The filling is ready when the center has set and no longer jiggles when the pan is gently shaken.
Transfer the pan to a wire rack and allow the bars to cool completely to room temperature.
Once cooled, refrigerate the bars for a minimum of 6 hours or leave them refrigerated overnight so the filling becomes sufficiently firm for clean slicing.
After the bars are thoroughly chilled, grasp the foil overhang and carefully lift the entire dessert from the pan. If any edges adhere to the sides, loosen them gently with a knife before lifting.
Place the slab on a cutting board and cut it into individual bars. Serve chilled, adding whipped cream to each portion if desired.