1. In a 350° regular or convection oven, bake nuts in an 8- or 9-inch-wide pan until lightly browned under skin, about 15 minutes. Pour onto a towel; fold towel to enclose nuts and rub packet between your hands to remove as much brown nut skin as possible. Let cool. Lift nuts from towel, leaving skins behind. Whirl nuts in a food processor or blender with liqueur until a smooth paste forms; scrape container sides as needed.
2. In a 1- to 2-quart pan over high heat, stir whipping cream until boiling. Remove from heat; add chocolate and stir until sauce is smooth. Measure 1/4 cup chocolate sauce into a microwave-safe bowl; set aside for garnish. Add hazelnut-liqueur paste to chocolate sauce in pan, nest in ice water, and stir until well mixed. Stir often until mixture cools to room temperature, 3 to 4 minutes.
3. In a deep bowl with mixer on high speed, beat egg whites and cream of tartar until they form a thick foam. Continue to beat, gradually adding granulated sugar and until whites hold distinct peaks.
4. Stir about 1/4 of the whipped whites into the chocolate-hazelnut mixture. Scrape chocolate-hazelnut mixture into bowl with remaining whites and fold gently until blended.
5. Lay crêpes on a counter, pale sides up and slightly apart. Mound equal portions of chocolate-hazelnut mixture in the center of crêpes and quickly fold all crêpe sides over filling to enclose, making packets 3 to 3 1/2 inches square. Lay packets seam side down.
6. Butter 6 shallow ramekins (5- to 6-in.-wide rounds or 4- by 6-in. ovals) and lay 1 filled crêpe, seam down, in each dish. Or arrange crêpes slightly apart in a buttered 10- by 15-inch pan.
7. Bake in 400° regular oven or 375° convection oven until centers barely jiggle when lightly touched, 10 to 15 minutes. If reserved chocolate sauce in bowl has solidified, warm in a microwave oven on full power (100%) for about 1/2 minute, stirring once or twice.
8. Serve dessert in ramekins or use a wide spatula to transfer to plates. Sift powdered sugar over crêpes. Top with ice cream and chocolate sauce.