Whisk together two fresh egg yolks with a dash of white wine vinegar in a large heat-proof bowl.
Place over a saucepan of simmering water (do not to let the bowl touch the water) and whisk until lukewarm.
Add 100g/3½oz melted butter to the egg yolks very, very slowly, beating continuously. Once the mixture has thickened, remove from the heat and season with salt and pepper.