Place the vinegar in a small pan with 2 tbsp cold water and the peppercorns. Bring to the boil for 30 seconds, then strain into a heatproof bowl, cool slightly.
Place the bowl over a pan of boiling water, and whisk in the egg yolks until they double in volume and become opaque in colour.
Add the hot clarified butter in a thin stream and whisk until thic