1. Heat a pan of hot water to a gentle simmer. Put the butter, egg yolks and, finally, the white wine vinegar in a Pyrex or stainless steel bowl over hot water and start whisking it together. The bowl shouldn’t touch the water beneath it. Season with Knorr Aromat.
2. Keep on whisking until it gets to the consistency you want – this will take about five minutes. When the whisk leaves a ‘trail’ in the sauce, that’s when you know it’s cooked.