Beat the butter with an electric whisk until pale and fluffy. Finely grind the pink peppercorns using a pestle and mortar. Stir into the butter with the lime zest and a pinch of salt, if you like, then whip in the lime juice. Spoon into a dish or shape using a waxed wrap or parchment and chill until needed. Will keep for one week. Serve with white fish.