Whisk together the milk, sweetener (if using), and vanilla extract.
Sprinkle the chia seeds over the milk and whisk in. Whisk immediately upon contact. There shouldn't be any seed clumps. If you see clumps, keep whisking.
Pour the chia milk into small mason jars or Tupperware containers and place in the fridge.
Allow it to set. It will turn into a thicker consistency. I usually leave it overnight.