Crispy Salt and Vinegar Smashed Potatoes

mixed baby potatoes (Yukon Gold, red, etc.) 1 Tablespoon kosher salt, plus addt’l for sprinkling 2 Tablespoons unsalted butter, melted 2 Tablespoons olive oil 2 Tablespoons white vinegar 2 Tablespoons chopped fresh chives Freshly ground black pepper