Cover the potatoes with boiling salted water in a large saucepan, add a sprig of mint and bring back to the boil. Reduce the heat, partly cover and simmer for 20–25 minutes, or until the potatoes are cooked. Drain well.
Melt the butter in the saucepan used to cook the potatoes and stir in the parsley. Return the potatoes to the saucepan and toss them gently in the hot parsley butter.
Transfer the potatoes to a heated serving dish, garnish with the sprigs of mint and parsley, if using, and serve immediately.