To a saucepan, add the erythritol, salt, and cocoa powder. Whisk to remove any lumps.
Add the half-and-half and egg yolks, and whisk to combine.
Turn the heat to medium and add the butter. Whisk constantly until the mixture begins to boil. This will take 15-20 minutes. Do not leave the the filling unattended or it will scorch.
When the mixture begins to bubble, add the xanthan gum 1/4 teaspoon at a time, whisking after each addition. Let the mixture bubble and thicken as you keep whisking. It should thicken like a pudding. Do not overcook.
Remove the pan from the heat and stir in the vanilla. Pour the filling into your pie shell.
To Make The Meringue:
Add the egg whites to the bowl of a mixer. With the mixer on high, beat the egg whites with the cream of tartar and vanilla extract until they become slightly firm.
With the mixer still on high, add the erythritol, a little a time. The meringue is done when it is firm and you can use the whisk to make peaks. It should hold its shape when done.
Top the pie with the meringue, making sure to seal the edges of the pie well. Use a knife or spatula to make peaks in the meringue.
Bake the pie for 5-10 minutes, or until the meringue is golden brown and set.