Mix the chocolate base: In a bowl or jar, whisk together the milk, cocoa powder, maple syrup, vanilla extract, and salt until the cocoa is fully incorporated.
Add the chia seeds: Stir in the chia seeds until evenly distributed.
Stir again: Let the mixture sit for 5 minutes, then stir again to break up any clumps.
Refrigerate: Cover and refrigerate for at least 2 hours, or overnight, until thickened.
Serve: Stir before serving, adding a splash of milk if needed to reach your preferred consistency.