3
Put another small pan, a quarter filled with boiling water, over a low heat. Place a heatproof mixing bowl over the pan of gently simmering water and add the egg yolks. Whisk in the vinegar using an electric or balloon whisk, and slowly drizzle in the melted butter, whisking vigorously all the time. The sauce should become smooth and thick. Remove from the heat, whisk in the lemon juice to taste and season with salt and pepper. Cover with cling film and set aside