In a large mixing bowl, combine the almond flour, erythritol, coconut flour, walnuts, baking powder, ground chia, pumpkin pie spice, and salt. Mix until all ingredients are well combined.
Place the egg whites in a mixing bowl and use an electric mixer to whip until stiff peaks form.
In a third large mixing bowl, beat the egg yolks for about 30 seconds. Add the butter, vanilla extract, apple cider vinegar, almond milk, and pumpkin puree. Whisk until all ingredients are well combined.
Add the dry ingredients to the egg yolk mixture and using a rubber spatula, stir to combine.
Gently fold the whipped egg whites into the mixture, being careful not to overmix and deflate the egg whites.
Divide the mixture evenly between 6 wells of a muffin pan, silicone muffin pan, or silicone muffin cups.
Bake for 25 to 30 minutes or until the tops are lightly browned and you can insert a toothpick in the center and remove it without anything sticking to it. (Start checking around the 22 minute mark)
For the marshmallow frosting:
Set up a double boiler by placing a heatproof bowl on top of a saucepan filled with about 1 inch of simmering water. Make sure that the bowl is not touching the water and is suspended above the water line by the rim of the pan.
Place the egg whites, erythritol, cream of tartar and salt in the bowl and using an electric mixer or electric whisk, whisk for 4 minutes.
Remove the bowl from the heat and continue whisking or transfer the mixture to a stand mixer for 7 to 9 minutes until the frosting is thick. Once the frosting is nice and thick, gently fold in the vanilla extract.
Spoon the frosting into a piping back and pipe each cupcake with the marshmallow frosting.
Sprinkle each cupcake with a tiny pinch of pumpkin pie spice before serving. Store leftovers in the refrigerator for up to 3 days.