Preheat your oven to 350°F (175°C). In a medium bowl, combine the almond flour, melted butter, erythritol, cinnamon, and a pinch of salt. Stir until the mixture resembles wet sand.
Press the crust mixture firmly and evenly into the bottom and up the sides of a 9-inch pie dish. Use the back of a spoon or a flat-bottomed cup to pack it tightly. Par-bake the crust for 10 minutes, then remove from the oven and set aside.
While the crust bakes, whisk together the pumpkin puree, eggs, heavy cream, erythritol, pumpkin pie spice, vanilla extract, and salt in a large bowl until completely smooth.
Pour the filling into the par-baked crust and spread it evenly.
Bake for 45 to 50 minutes, until the edges are set and the center has just a slight jiggle. If the crust edges begin to darken too quickly, tent them loosely with foil.
Remove the pie from the oven and let it cool to room temperature on a wire rack. Then refrigerate for at least 4 hours or overnight to set completely.
Slice and serve chilled with a dollop of unsweetened whipped cream if desired.