Place egg yolks in a tall narrow container that the blender stick fits in all the way to the base.

Add water, lemon juice, cayenne pepper and salt. Blitz briefly to combine.

Melt butter in a heatproof jug until hot. Be very careful to ensure it doesn't explode if using microwave.

Let butter stand for just 15 seconds or so until the milky whites settle at the bottom of the jug.

With the blender stick going on high, slowly pour the butter in a thin stream into the eggs over around 45 seconds, leaving behind most of the milky whites in the butter.

Once all the butter is in, the sauce should be thick, creamy, smooth and pale yellow. Blitz for a further 10 seconds, moving the stick up and down.

If too thick, mix in warm tap water 1 teaspoon at a time.

Adjust salt if necessary and serve immediately, or keep warm until required.