Start by melting the butter in a saucepan over a gentle heat. When all butter is melted then remove the saucepan from the heat.
Take a new saucepan and heat some water and place a stainless steel bowl or glass on top, and keep on a gentle simmer.
Pour in the white wine vinegar and lemon juice then add the egg yolks and whisk continuously until the sauce reaches a light and creamy consistency. Be very careful at this stage because the line between creamy and scrambled is very fine.
Now remove the bowl from the saucepan then pour in the melted butter, whilst continuing to whisk at all times.
If the sauce is still a little thick after adding all the butter, whisk in some cold water or squeeze in a lemon. Season to taste.