Combine the white wine vinegar, white wine, tarragon stalks, peppercorns and shallot in a small saucepan. Bring to a simmer over a moderate heat and reduce until you have 2 tbsp of the reduction left. Set to one side to cool slightly.
Place the butter in a small saucepan and melt over a low heat. Pour the golden pure butter fat into a heatproof jug and discard the milk solids. Place the egg yolks and the reduction in a heatproof bowl and set over a large saucepan of simmering water. Whisk until you have a fluffy sabayon; when the whisk leaves trails in the mixture.
Remove from the heat and whisk in the clarified butter in a slow, steady stream until you have a thickened sauce. Stir in the chervil, then season to taste. Spoon into a small serving pot and garnish with a sprig of chervil Serve immediately.
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