Boil the potatoes in a large pot of salted water for about 10 minutes until tender, drain and cut in half.
Melt 1 tablespoon of the butter in a frying pan and sauté the pancetta over a medium heat for about 3 minutes until just browning. Add the rest of the butter and cook until the pancetta is golden and the butter is just starting to go brown, about 2 minutes. Tip in the shallot and rosemary sprig and cook for a further 2 minutes. Remove the pan from the heat and discard the rosemary.
Whisk the vinegar and mustard into the pan, season with salt and pepper and tip in the potatoes. Toss to completely coat them. Sprinkle with chives if using and serve warm or at room temperature. Serves 4-6.
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