Melt the butter in a small pan.
Put the egg yolks into a heatproof bowl over a pan of gently simmering water and whisk with a tablespoon of lemon juice and mustard.
Whisking slowly (reduce chance of splitting), pour the melted butter into the egg mixture, until well combined, adding a splash of water to loosen, if needed.
Whisk in a splash of vinegar and season to perfection.
Turn the heat off and keep warm over a pan of water, stirring occasionally, and loosening with an extra splash of water when needed.