Sift the unsweetened cocoa powder into a medium bowl to get rid of the clumps. Add the maple syrup, vanilla extract and a pinch of salt. Whisk until smooth. Add the milk and whisk again until smooth. Pour the chia seeds on top of the mixture and whisk to combine. Cover the bowl and refrigerate until the mixture thickens and becomes pudding like, at least 4 hours, or preferably overnight. Whisk a few times while the pudding is setting in the refrigerator. Serve the pudding with toppings of your choice.