Line an 8x8 or 9x9 square baking pan with parchment paper, leaving two opposite sides overhanging to create easy lifting handles.

Stir together the graham crumbs, brown sugar, pumpkin pie spice, and salt in a big mixing bowl. Pour in the melted butter and mix thoroughly until every crumb is coated.

Transfer the mixture into the pan and press it into a firm, even layer using the bottom of a measuring cup or a flat glass. Put it aside.

Pour the cold heavy cream into a large chilled bowl and beat until stiff peaks form. Keep the whipped cream in the refrigerator while you work on the filling.

In another large bowl, beat the cream cheese with the granulated sugar and salt until perfectly smooth and free of lumps. Add the vanilla, lemon juice, pumpkin puree, and pumpkin pie spice, then beat again until the mixture is silky and uniform.

Use a rubber spatula to fold the reserved whipped cream into the pumpkin cream cheese mixture in a gentle circular motion, scraping the bottom, until fully incorporated with no streaks.

Spoon the filling over the prepared crust and smooth the top with an offset spatula. Cover the pan and refrigerate for at least 8 hours, preferably overnight. For neat slices, freeze for 1 hour before cutting.

Lift the set cheesecake out of the pan using the parchment overhang and transfer it to a cutting board. Trim any uneven edges, then cut into 12 even bars in a 4 by 3 grid, wiping the knife clean after each slice.

In a chilled bowl, whip the cream, powdered sugar, and vanilla until stiff peaks form. Place the cream in a piping bag fitted with a large star tip (like Wilton 1M) and pipe a decorative swirl onto each bar. Finish by dusting the tops with pumpkin pie spice and serve chilled.