1. Preheat oven to 325°. In a large bowl, whisk together sugar, cocoa, and salt. Stir in chocolate and walnuts. Add egg whites and vanilla; stir just until incorporated (don't over-mix).
2. Drop dough by the tablespoon, 2 inches apart, onto three parchment-lined rimmed baking sheets. Bake until cookie tops are crackled but still slightly moist (15-18 minutes), rotating sheets halfway through. Transfer sheets to wire racks; let cookies cool completely.
3. Freeze the cookies for at least 30 minutes. Place 3 tablespoons of sorbet on the flat side of a cookie and top with another cookie. Repeat with remaining cookies. Store the ice cream sandwiches in the freezer in an airtight container until ready to eat. Can be made up to 2 days ahead.