In the top of a double boiler or in a large heatproof bowl set over a saucepan of simmering water, whisk the egg yolks with 1 1/2 tablespoons water, whisking vigorously, until the mixture thickens, about 4 minutes. Remove from the heat, and stir in the lemon juice.
Slowly whisk in the melted butter until thickened. Season with salt. Serve the sauce immediately, or keep warm over very gently simmering water, whisking occasionally.
