1. Bring a large saucepan of salted water to a boil. Add the butter beans and cook until they are tender, 8 to 10 minutes. Drain and rinse the beans under cold running water. Drain well and pat the beans dry.
2. In a large bowl, whisk the olive oil with the lime juice and buttermilk; season the dressing with salt and pepper. Add the beans, cover and refrigerate for 2 hours. Fold in the mint before serving.