Increase the oven temperature to 375℉. Scatter the hazelnuts on a baking sheet and roast for about 10 minutes. Let cool completely. Remove the skins. This should happen easily with your fingers; alternately you can rub them in a clean, fuzz-free dish towel. See notes for more advice if needed. Coarsely chop the skinned hazelnuts and set aside.
In a small or medium saucepan, bring the cream and coffee just to a simmer. Add all but ¼ cup of the chopped chocolate, plus the butter and salt. Let stand for 2 minutes, then stir continuously with a rubber spatula until smooth. Pour the chocolate mixture into the tart shell. Scatter the hazelnuts on top, gently pressing them into the chocolate.
Melt the remaining ¼ cup chocolate in the microwave in 20-second bursts. To avoid it seizing on you, melt it almost all the way. Stop microwaving when it’s about three-quarters of the way melted. Pull it and stir vigorously to melt it completely.
Immediately use a spoon to drizzle it all over the top of the tart in pretty stripes or circles (circles are way easier!).
Refrigerate until firm, at least 1 ½ hours, but overnight is better. Slice into wedges and serve.