Pour the milk into a bowl or a jar with a lid. Add the chia seeds, cocoa powder, sweetener, vanilla extract, and salt.
Whisk everything together until the cocoa is fully blended and the chia seeds are evenly distributed.
Let the mixture rest for about 5 minutes, then stir again to break up any chia seed clumps and ensure a smooth texture.
Cover the container and refrigerate for at least 6 hours, preferably overnight, until the pudding becomes thick and creamy.
Before serving, give the pudding one final stir. If it has thickened more than you'd like, mix in a splash of milk until it reaches your preferred consistency.
Finish with your favorite toppings and enjoy chilled.