1.Trim and wash livers; cut in half. Combine livers in small bowl with port; cover. Stand 2 hours; strain.
2.Melt a quarter of the butter in medium frying pan; cook onion and garlic, stirring constantly over medium heat about 3 minutes or until onion is soft. Add livers to pan; stir constantly about 5 minutes or until livers are just changed in colour. Stir in tarragon and brandy; bring to the boil. Reduce heat; simmer, uncovered, about 3 minutes or until livers are tender.
3.Melt a quarter of the butter in saucepan. Blend or process liver mixture until smooth. Add tomato paste and sour cream; blend until combined. While motor is operating, gradually add melted butter.
4.Pour pâté into four ½-cup (125ml) serving dishes; garnish with sprig of herbs or bay leaf, if desired. Cover; refrigerate 2 hours.
5.Coarsely chop remaining butter; melt in small saucepan over low heat, without stirring. Stand a few minutes, then use spoon to carefully remove and discard whitish-coloured scum floating on surface. Carefully pour remaining clear liquid into jug, leaving whitish milky deposits in the pan; discard deposits.
6.Gently pour a thin layer of clarified butter over pâté; refrigerate overnight.