Preheat your oven to 325°F (165°C). Wrap the outside bottom of a 9-inch springform pan with foil to prevent leaks. In a bowl, mix together the almond flour, melted butter, erythritol, cinnamon, and salt. Press firmly into the bottom of the pan.
Par-bake the crust for 10 minutes until lightly golden. Remove from the oven and let it cool while you prepare the filling. Reduce the oven temperature to 300°F (150°C).
In a large bowl, beat the cream cheese with an electric mixer on medium speed until completely smooth, about 3 minutes. Add the erythritol and beat until combined.
Add the pumpkin puree, pumpkin pie spice, and vanilla extract and mix until incorporated. Add the eggs one at a time, beating on low speed after each addition just until blended. Stir in the sour cream until smooth.Avoid overmixing, as this can cause cracks. The batter should be silky and uniform.
Pour the filling over the cooled crust and smooth the top. Place the springform pan in a larger roasting pan and pour hot water into the roasting pan until it comes about 1 inch up the side of the springform.
Bake in the water bath for 60 to 70 minutes, until the edges are set and the center has a gentle jiggle. Turn off the oven, crack the door open, and let the cheesecake cool inside for 1 hour.
Remove the cheesecake from the water bath and refrigerate for at least 6 hours or overnight.
Run a thin spatula around the edge before releasing the springform ring. Slice and serve chilled with a dollop of whipped cream if desired.