Place the cacao butter in a small saucepan over low heat (or use a double boiler) and heat gently, stirring, until just melted; then remove from the heat.
In a bowl, combine the cocoa powder and powdered sweetener; for a smoother result, pulse them in a high-speed blender first until very fine.
Stir the cocoa powder and powdered sweetener into the melted cacao butter until fully combined and smooth.
Fold in any optional extras you want (orange zest, cinnamon, salt, chili, granola, or nuts/seeds).
Pour the mixture into a silicone chocolate mould or spread it onto parchment-lined tray to make bark or clusters.
Freeze for about 5 minutes, or until set, then remove from the mould or break into pieces as desired.