Set a small heatproof bowl over barely simmering water. The bowl base should not touch the water.
Whisk yolks water and lemon juice until they thicken and leave faint trails.
Drizzle in warm butter in a thin stream while whisking. Keep the steam gentle. If it thickens too fast lift the bowl for a few seconds.
Season with salt and pepper. Whisk until glossy and smooth. Serve right away.