Add the chia seeds, cinnamon, brewed coffee, milk, vanilla extract and sweetener and to a small dish or container and whisk well.
Refrigerate overnight or for at least 6 hours to thicken. If the mix is still thin, add 1-2 more tbsp of chia seeds to help thicken it. The longer it sits, the thicker it will get.
Once chilled, whisk again to break up any chia clumps.