For the best texture, boil the cocoa-sugar mixture for a full 60 seconds; underboiling leads to soft cookies, overboiling can make them dry. Creamy peanut butter yields the smoothest results, but natural peanut butter works if well stirred. Store in an airtight container at room temperature for up to 5 days or refrigerate for up to 1 week. Freeze for up to 2 months.