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1
Place the egg yolks and the tablespoon of cold water in a heatproof bowl. Set the bowl over a bain-marie held at 55–62°C — the water below must not boil. Whisk the yolks continuously until they triple in volume and form a thick ribbon that holds its shape for three seconds when the whisk is lifted. This is the sabayon stage: the egg proteins have unfolded sufficiently to receive and stabilise the incoming fat. Done by sight and touch: the ribbon falls slowly and does not dissolve back into the mass.
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2
This is where the sauce lives or dies. Remove the bowl from the bain-marie. Begin adding the clarified butter — held at 55–60°C — in the thinnest possible stream, whisking without pause. The first 50 ml must be added drop by drop, exactly as with a mayonnaise: the emulsion is at its most fragile at the outset and will not recover if butter is introduced faster than the yolks can absorb it. Once the emulsion has visibly thickened and the butter is being taken up cleanly, the stream may flow more freely. Done by sight: the sauce tightens with each addition and takes on a glossy, creamy body.
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3
Season the finished sauce with lemon juice, fine salt, and a pinch of cayenne. Stir to combine. Verify the balance by taste — butter, lemon, and egg should be in equal harmony — and check consistency by drawing the back of a spoon through the sauce: it should coat with a thick, creamy film that does not run. Done by taste and sight.
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4
Serve immediately. The sauce cannot be reheated, refrigerated, or held for more than 30 minutes. Should early curdling occur before service, remove the bowl from the heat at once and whisk in an ice cube to arrest protein coagulation.