Gently heat butter over low heat in a small saucepan until just melted. (Do not boil.) Set aside.
Place egg yolks in a blender container. Add lemon juice and salt. Blend together briefly, then drizzle in melted butter as blender is running on low-medium speed. When mixture has thickened, add the whey and pulse briefly. Use immediately or allow to sit at room temperature for 6 to 8 hours to lacto-ferment (only if you are using pastured egg yolks; if not, refrigerate promptly). Makes about 1 cup of sauce or enough to cover 6 poached eggs. Store any leftovers in the refrigerator and reheat in a double boiler, being careful to heat only until just warmed through.