The slight bitterness of orange zest and a sprinkling of salt round out the richness of the chocolate and hazelnuts.
Line a small (9-by-13-inch) baking sheet with aluminum foil; refrigerate. With a vegetable peeler, remove zest from half the orange. With a sharp knife, cut zest into very thin strips, each about an inch long.
Place chocolate in a heatproof bowl set over (not in) a pan of simmering water. Stir until melted and completely smooth. Mix in half the zest and hazelnuts. Immediately, pour onto prepared baking sheet; with a flexible rubber spatula, spread into an 8-by-10 inch rectangle about 1/4-inch thick. (To help spread chocolate, tip baking sheet up and down.) Sprinkle with remaining zest, nuts, and coarse salt.
Refrigerate until firm, about 30 minutes. To serve, break into pieces. To store, transfer to an airtight container, and refrigerate, up to 3 days.