Rocher may mean "rock" in French, but with crisp outsides that yield to soft, chewy centers, these cookies hardly resemble their namesake. The batter must be refrigerated overnight so that it is firm and thoroughly chilled before baking.
Pulse toasted hazelnuts, confectioners' sugar, and salt in a food processor until finely ground (mixture should be powdery). Add egg whites and orange zest and juice, and pulse until combined. Transfer batter to an airtight container, and refrigerate overnight.
Preheat oven to 350 degrees. Nest 2 baking sheets (to prevent bottoms of cookies from browning too quickly), and line top sheet with parchment. Drop tablespoons of batter onto lined sheet, spacing them 1 inch apart. Shape each into a mound, top with 2 hazelnut halves, and sprinkle generously with sanding sugar. Refrigerate remaining batter.
Bake cookies, rotating sheets from front to back halfway through, until golden brown, 20 to 23 minutes. Let cool completely on a wire rack. Repeat with remaining batter.
Cookies can be stored in an airtight container at room temperature for up to 1 week.