Bring a pot of salted water to a rolling boil. Add the green beans and blanch for 3-4 minutes until vibrant green and tender-crisp. Transfer to a bowl of ice water to halt cooking.

In the same pot, add small waxy potatoes, cover with water, bring to a boil, then simmer for 15-20 minutes until fork-tender. Let cool for 15 minutes.

In a bowl, combine ¼ cup balsamic vinegar, 1 tablespoon Dijon mustard, and 1 minced garlic clove. Whisk until well blended. Slowly drizzle in ⅓ cup olive oil while whisking to emulsify.

Slice cooled potatoes in half and place in a large mixing bowl with the blanched green beans. Pour balsamic dressing over and toss gently to coat evenly.

Sprinkle with chopped chives and black pepper. Let sit for 15-20 minutes at room temperature to meld flavors before serving.