Preheat the oven to 350 F (180 C). Combine the ingredients for the pie crust, including the almond flour, coconut flour, 1/4 cup powdered erythritol, 3 tbsp cocoa powder, xanthan gum, and kosher salt. Add in the melted butter and stir to moisten all dry ingredients.
Press the pie crust into a pie dish. Make sure to press the crust up the sides of the pie dish. Then bake the crust for 10-12 minutes or until the crust is slightly puffed and cooked through. Allow the crust to cool completely before filling. To speed up the process, allow the crust to cool at room temperature for 10 minutes or so before placing it into the refrigerator.
While the crust is cooling, combine the ingredients to make the filling. Start a small pot of boiling water on a stove. Then combine the canned coconut milk and sugar-free chocolate chips in a heatproof bowl. Place the bowl over the boiling water. Stir until the chocolate is completely melted. Then add 2 tablespoons of cocoa powder, 1 teaspoon of vanilla extract, and ½ cup powdered erythritol. Whisk well to combine.
Then pour the filling into the chilled crust. Allow the pie to chill overnight in the refrigerator. Doing so will ensure that the filling is firm enough to slice.
The next day, make the chocolate buttercream. Beat ½ cup powdered erythritol, ½ stick of room temperature butter, 2 tablespoons of cocoa powder, ½ teaspoon of vanilla extract, and 1 tablespoon of almond milk together until light and fluffy. It is best to do this in the bowl of a stand mixer, making sure to scrape down the sides several times.
Fill a piping bag fitted with a star tip (#823) with the chocolate buttercream. Pipe 8 dollops around the edges of the pie. Do so in a way that when you cut the pie, each slice will have a bit of frosting on it. Cut the pie and serve chilled!