Divide the milk, Greek yogurt, protein powder, and salt evenly between two jars. Stir each jar until smooth and well combined. Add 2 tablespoons of chia seeds to each jar and stir well.
Let the jars sit for 5–10 minutes, then stir again to prevent the chia seeds from clumping.
Stir 1 tablespoon of Biscoff spread into each jar until well combined.
Cover and refrigerate for at least 4 hours, preferably overnight, until thick and creamy.
In the morning, stir each jar. If the pudding is too thick, add a splash of milk and stir until you reach your desired consistency.
Top each jar with a drizzle of warmed Biscoff spread and, if desired, crushed Biscoff cookies.