Add the chia seeds, instant coffee powder and pour the remaining milk on top (so you don't need to stir at this point).
Place the cup in the refrigerator and let it soak for about 4 to 6 hours.
After that, add the heavy cream and sweetener also to the cup (also add cinnamon and vanilla if using), mix well and place it back in the refrigerator and chill for another 4 to 6 hours.
After chilling for enough hours, stir well and enjoy! (top with a little more cinnamon powder if desired right before serving)