Drain and rinse the hazelnuts from the soaking water, then add to a high powered blender with filtered water. Blend until completely smooth and milky looking. If you've never made nut milk before, keep in mind you need a high powered blender like a Vitamix in order to get the nuts blended thoroughly.
Strain by hand through a nut milk bag, a double layer of cheesecloth, or you can use a pair of cheap brand new panty hose. Cut one leg off! Pour the milk through whatever you're using over a large bowl and squeeze out the liquid by hand until the pulp is dry and there is no more liquid left.
Over medium low heat, add the chocolate, sugar, cacao powder, sea salt and about 1 cup of the hazelnut milk to a large sauce pan and whisk constantly until it melts and is smooth. Then add in the rest of the hazelnut milk and bring to a low simmer. Heat it through and serve hot with marshmallows or coconut whip on top!