Prepare the Shortbread Crust: Preheat your oven to 350°F (175°C). Line a 9 x 9 inch baking pan with parchment paper, allowing the sides of the parchment to extend over the sides of the pan for easy removal. Set aside.
In a stand mixer fitted with the paddle attachment, cream the butter, granulated sugar, and brown sugar for about 3 minutes, or until light and fluffy. Scrape down the sides of the bowl as needed.
Add the egg and vanilla, mixing well until fully incorporated.
Add the flour, baking powder, cinnamon, and salt. Mix until the dough just begins to pull away from the sides of the bowl. Avoid over-mixing.
Press the shortbread dough evenly into the prepared pan. Use the bottom of a measuring cup to press it into the corners and level the top.
Bake for about 15 minutes, or until the edges are lightly golden brown and the center is set but not fully cooked. Remove from the oven and let cool slightly.
Prepare the Pumpkin Filling: In a large bowl, whisk together the pumpkin puree, heavy cream, eggs, brown sugar, granulated sugar, and vanilla until smooth.
Add the cornstarch, pumpkin pie spice, cinnamon, and salt, whisking until fully combined.
Pour the pumpkin filling over the pre-baked shortbread crust, spreading it evenly.
Carefully transfer the pan onto a large baking sheet and place it in the oven. Bake at 400°F for 15 minutes. Then, reduce the temperature to 350°F and bake for an additional 40 minutes, or until the edges are set and the center is slightly wobbly. Do not overbake.
Remove from the oven and cool on a wire rack for 45 to 60 minutes. Transfer to the refrigerator to chill for at least 3 hours or overnight.
Whipped Cream Topping: In a stand mixer with the whisk attachment (or using a hand mixer), whip the heavy cream and powdered sugar on medium-high speed until firm peaks form. Keep chilled until serving.
Slice the chilled bars, top with swirls of whipped cream, and a sprinkle of nutmeg. Serve and enjoy!