How can one make dry fine hazelnut flour?
Over half of the weight of hazelnuts consists of oil.  So just:


buy / pick the hazelnuts yourself
buy yourself a decent nut oil press, (the higher the pressure the more oil you will remove) 
press the nuts


and whatever is left after you've pressed all the oil out of the nuts is very dry though still coarse so the only thing left to do is just use a grain grinder to grind that into the smoothest most wonderful flour you've ever seen and tasted.

Tip: if you're only interested in the nut flour, start selling the hazelnut oil as it's a fantastic product to make all kinds of fried desserts like fried chocolate to offset the 2 new items you had to buy