Set oven to 350F

Line a baking sheet with a silicone mat, or parchment paper, and set aside.

Spread the nuts on a separate dry baking sheet and roast in the oven for 15 minutes, giving the pan a shake a couple of times during cooking to redistribute the nuts. Let them cool on the pan.

Put the white chocolate (and the coconut oil, if using) in a microwave safe bowl and microwave for a minute. Let sit for a minute, then stir. if it is not completely melted (it probably won't be) microwave in further 15 second bursts, stirring well in between, until most of the lumps are gone. Keep stirring until the chocolate is smooth. Stir in the hazelnuts, and then turn out onto the lined baking sheet. Spread the chocolate out into a thin rectangle. The exact size if variable, mine was about 12x7. The important thing is that there are no bare spots. Note: if you have trouble melting chocolate, see the tips in the post, above.)

Put the tray into the refrigerator to harden for about 30 minutes, then cut into pieces. The bark will shatter a bit when you cut it, that's to be expected.

You can store the bark at room temperature or in the refrigerator, and it will theoretically last for a month. But in practice, it will probably be gone by the end of the day ;)