Rinse the quinoa thoroughly in a fine-mesh strainer under cold water for at least 30 seconds to remove any bitter coating.
Add the rinsed quinoa, almond milk, cocoa powder, sugar-free sweetener, vanilla extract, cinnamon, and salt to the slow cooker.
Whisk everything together until the cocoa powder is fully incorporated and no lumps remain.
Cover and cook on low for 2 hours, stirring once halfway through if possible.
After 2 hours, stir in the chopped sugar-free dark chocolate until melted and creamy.
The pudding will thicken as it cools, so let it sit for 10-15 minutes before serving.
Serve warm or transfer to individual containers and refrigerate for up to 5 days.