Whisk the egg yolks and lemon juice together in a heavy bowl until lightened. In the meantime, bring a saucepan of water to barely simmering. Put the bowl on top (bowl should not touch water) and continue whisking until eggs reach 160 degrees F. Remove from heat, but do not let the eggs sit. Working quickly, add the mustard and either:

• Whisk by hand: Whisk in the butter, drizzling slowly until sauce is thickened and doubled in volume, six to eight minutes.
• Use a stick blender: Add yolks to a mason jar or other sturdy tall container and use a stick blender as the butter is drizzled in, blending until doubled in volume.
• Use a standard blender: Add the yolks to a blender and with blender running, drizzle the butter, continuing to blend until doubled in volume.

When finished, blend in the pinch of cayenne and taste for salt, adding a pinch if needed.

Keep warm until serving.