Bring 1 inch of water to a simmer in a large saucepan over medium-low heat. Set out a medium stainless steel bowl. In a small saucepan, melt the butter over medium heat.
Using a blender, mix the egg yolks and lemon juice on high speed for 20 seconds. With the machine on, gradually pour in the boiling water, then the warm butter. Add 1/2 tsp. salt and 1/4 tsp. pepper, blending on high speed for 1 minute.
Pour the egg-butter mixture into the reserved bowl and set over the simmering saucepan of water. Whisk the mixture constantly until steaming-hot and thick enough to coat a spoon, about 5 minutes.