You may substitute other green bean varieties, but since most other types are thicker, increase the cooking time by 2 to 3 minutes (until the beans are tender yet crisp).
Add haricots verts and cook until tender but still crisp, about 1 minute. Drain immediately and place in ice water until chilled. Drain beans well and set aside.
Meanwhile, add balsamic vinegar to shallots in a medium bowl; let sit 30 minutes. Stir in mustard until combined. Gradually whisk in olive oil and hazelnut oil until blended. Whisk in chives, 1/4 teaspoon salt, and pepper until combined.
Toss in beans and hazelnuts until evenly coated with dressing and serve.