Place the egg yolks and water into a small bowl that fits snugly over a pan of gently simmering water and whisk well until thickened. Add the mustard and lemon juice and whisk well.
Gradually add all the melted butter in a steady stream until the mixture is thick and emulsified. Season with salt and freshly ground black pepper to taste. Set aside and keep warm.
In a saucepan, boil some salted water. Place the broccoli into the boiling salted water for four minutes, or until just cooked. Drain well.
To serve, pile the broccoli onto a serving plate and pour over the hollandaise sauce.