Melt the butter and pour it into the large mixing bowl. Add in cocoa powder, sweetener, and almond flour. Knead the mixture until the dough is well mixed.
Roll the dough between two parchment papers into one big thin circle. Transfer the flat dough into the 9-inch pie pan to cover the whole pan. Refrigerate while you make the mousse filling.
For the filling: Melt dark chocolate with coconut oil in a bowl until smooth and completely melted using a double-boiler or microwave. Add in mashed avocado, heavy cream, sweetener and cocoa powder. Mix well.
In another bowl, combine gelatin with water. Wait for the mixture to thicken then add it into the chocolate-avocado mixture. Mix well.
Spread the chocolate mousse over the crust and refrigerate for at least 30 minutes. Cut into 8 pieces and serve cold.