To soak the hazelnuts, place them in a glass or ceramic bowl or large glass jar, and cover with filtered water. Add 1 teaspoon Celtic sea salt and splash of fresh lemon juice or apple cider vinegar, cover the container with a breathable kitchen towel, and allow to soak at room temperature for 8 hours. (For more information on soaking read here.)
Drain, and discard the soaking liquid (do not use this to make the milk). Rinse the hazelnuts several times to remove the anti-nutrients and enzyme inhibitors.
Throw the rinsed hazelnuts, water, and salt in your blender, and blast on high for 30 to 60 seconds, until the nuts are completely pulverized. Use whole milk to maximize nutrition. Or strain for a smoother, more commercial-style milk for use in recipes.
To strain, place a nut milk bag or knee-high piece of sheer nylon hosiery over the opening of a glass bowl, jar or jug. Pour the milk into the bag, twisting the bag closed, and gently squeezing it to pass the liquid through. Empty the hazelnut pulp aside. You can dehydrate this for use in smoothies or to make crusts.
Rinse your blender container, and pour the strained milk back in. Add the vanilla, sweetener, and any flavorings, and blast again, until smooth and creamy.
Store the milk in a sealed container in the fridge. Activated hazelnut milk (made with soaked hazelnuts) will keep for 2 to 3 days in a very cold fridge. Un-soaked hazelnut milk will keep for about 5 days.