Place the hazelnuts in a bowl or blender container. Add the water, and allow the hazelnuts to soak in the refrigerator, covered, for at least 8 hours, or overnight.
Once the hazelnuts have soaked overnight, place in the blender and add the vanilla extract. Cover with lid and blend until consistently white in color and hazelnut is finely blended and ground.
Place a sieve (lined with a coffee filter), cheesecloth (secure with rubber band), or nut bag over a container of your choosing, and strain the hazelnut milk and separate the hazelnut pulp. Go slowly—you may need to do this step in batches depending on the size of the container and sieve. If you are using a coffee filter, this step could take as long as 30 minutes or longer.
Chill and store the hazelnut milk in a covered container in the fridge for up to 2-3 days (shake before using).
