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1
For the reduction, put the wine vinegar, water, peppercorns, bay leaf and mace in a small saucepan and bring to a simmer.
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2
Reduce the liquid by at least two-thirds, then strain to remove the peppercorns, bay and mace.
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3
To make the sauce, cut the butter into 1cm cubes.
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4
Put the egg yolks, a cube of butter and a small pinch of salt in a small bowl.
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5
Using a wooden spoon, cream the butter into the egg yolks, add 1/21 tsp of the reduction and stir to combine.
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6
Half-fill a roasting tin with water and set over a medium heat, to create a warm bain marie.
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7
Heat until the water is hand-hot (it should be comfortable to dip your fingers into).
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8
A bubble might occasionally rise to the surface, but if you have the water too hot, the sauce will curdle.
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9
Turn off the heat.
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10
Stand the bowl containing the egg yolk, butter and reduction mixture in the bain marie, off the heat, and stir until the egg yolks visibly start to thicken.
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11
Beat the remaining butter in, a cube at a time, making sure that the sauce has re-thickened before adding the next cube of butter.
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12
As more butter is added, the heat can be turned on to low (but take care that the water does not boil) and the butter can be added more quickly.
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13
If the sauce becomes very thick and appears greasy, add a little more reduction or cool water.
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14
Once all the butter has been added, remove the bowl of sauce from the roasting tin, taste it and adjust the seasoning.
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15
To achieve a balanced flavour, you may need to add more reduction, lemon juice, salt and ground white pepper.
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16
The sauce can be kept for about 30 minutes before serving, by standing the bowl in a warm bain marie, off the heat.
