Put the vinegar into a small pan with the peppercorns, mace, onion slice and bay leaf. Bring to the boil and reduce to 1 tbsp liquid. Dip the base of the pan in cold water to stop further evaporation; set aside.

Put the egg yolks into a heatproof bowl with 15 g (1/2 oz) butter and a pinch of salt. Beat until well combined, then strain in the reduced vinegar.

Put the bowl over a pan of barely simmering water and whisk for 3-4 minutes until the mixture is pale and beginning to thicken.

Beat in the remaining butter, a piece at a time, until the mixture begins to thicken and emulsify. Ensure each addition of butter is incorporated before adding the next. Do not allow the mixture to overheat or the eggs will scramble and split. Remove from the heat.

Whisk in the cream if using. Season the sauce with salt and pepper, and add a little lemon juice to taste. Serve at once.

If the sauce shows signs of curdling, add an ice cube and whisk thoroughly; the hollandaise should re-combine.

To make hollandaise in a food processor, melt the butter and let cool until tepid. Put the strained reduced vinegar, egg yolks and salt in the processor bowl and process for 10 seconds. With the motor running, add the melted butter in a thin steady stream through the feeder tube and process until emulsified. Finish as in stage 5, above.