When boiling the potatoes, begin timing as soon as they’re added to the water. To make ahead, the potatoes can be boiled, smashed, cooled, and refrigerated a day in advance; to finish, brush with oil and roast as directed. The vinaigrette can be made in advance except for the chilies, then covered and refrigerated until ready to use; bring to room temperature and add the chilies. Don’t let the potatoes cool completely before smashing. They are easier to flatten and they hold their shape better when warm.