Preheat the oven to 350 degrees F (177 degrees C). Line the bottom of a 9 in (23 cm) springform pan with parchment paper. (You can also try greasing well.)

To make the almond flour cheesecake crust, stir the almond flour, collagen or protein powder, and sweetener together.

Whisk together the melted butter and vanilla, then stir into the dry ingredients, pressing with the spoon or spatula, until well combined. The dough will be slightly crumbly.

Press the dough into the bottom of the prepared pan. Prick gently with a fork all over. Bake for about 12-15 minutes, until barely golden. Let cool at least 10 minutes.

Meanwhile, beat the cream cheese and powdered sweetener together at low to medium speed until fluffy. Beat in the pumpkin puree, pumpkin pie spice, cinnamon and vanilla. Beat in the eggs, one at a time. (Keep the mixer at low to medium the whole time; too high speed will introduce too many air bubbles, which we don't want.)

Pour the filling into the pan over the crust. Smooth the top with a spatula. (Use a pastry spatula for a smoother top if you have one that fits into the pan.)

Bake for about 40-50 minutes, until the center is almost set, but still jiggly.

Remove the cheesecake from the oven. If the edges are stuck to the pan, run a knife around the edge. (But, do not remove the springform edge just yet.) Cool the cheesecake in the pan on the counter to room temperature, then refrigerate for at least 4 hours (preferably overnight), until completely set. (Do not try to remove the cake from the pan before chilling.)

Serve with whipped cream and/or a sprinkle of cinnamon.