Vigorously whisk the egg yolks and lemon juice together in a stainless steel bowl and until the mixture is thickened. A small skillet can be used, as long as it fits over the bowl you plan to use in the next step.
Place the bowl over a saucepan containing barely simmering water. IMPORTANT: the water should not touch the bottom of the bowl.
Continue to whisk rapidly. Be careful not to let the eggs get too hot or they will scramble. Slowly drizzle in the melted butter and continue to whisk until the sauce is thickened. Stir in diced mushrooms.
Remove from heat, whisk in hot sauce and salt.
Cover and place in a warm spot until ready to use. If the sauce gets too thick, whisk in a few drops of warm water before serving.