Prepare your double boiler: Fill a small saucepan with about 5cm of water and bring it to a gentle simmer over medium heat. You want small, lazy bubbles rising to the surface—a vigorous boil is too harsh and will scramble your eggs. Place a heatproof glass bowl over the saucepan, ensuring the bottom of the bowl does not touch the water. The steam will gently cook the yolks.

Whisk the yolks and acid: Add the egg yolks, lemon juice, and cold water to the bowl. Whisk immediately and continuously until the mixture becomes pale, thick, and frothy. You'll see it increase in volume and lighten to a creamy, lemon-yellow colour. This should take about 1-2 minutes. The sound will change from a sloshing liquid to a thicker, more cohesive whisking sound.

Add the cold butter cube: Drop in one single, cold cube of butter. Whisk until it is fully melted and incorporated. This initial cold cube is the secret—it helps stabilise the emulsion and prevents the yolks from cooking too quickly. The mixture will look smooth and glossy.

Add the remaining butter, cube by cube: Continue adding the remaining butter cubes one at a time, whisking constantly. Wait until each cube is almost completely melted and incorporated before adding the next. The sauce will slowly thicken and become a luscious, pourable consistency. You'll feel the whisking become slightly heavier as the sauce emulsifies.

Check for the ribbon stage: After all the butter is incorporated, continue whisking for another 30-60 seconds. The sauce should be thick enough to coat the back of a spoon. When you lift the whisk, the sauce should fall back into the bowl in a thick, smooth ribbon that slowly blends back into the sauce. The colour will be a brilliant, buttery yellow.

Season and finish: Remove the bowl from the heat. Whisk in the fine sea salt and a tiny pinch of cayenne pepper. Taste and adjust the seasoning, adding a little more lemon juice if you prefer a sharper tang. The sauce should be warm, silky, and fragrant with butter.

Serve immediately: Hollandaise is best enjoyed straight away. If you need to hold it for a few minutes, place the bowl over the warm (but turned-off) water and stir occasionally. Do not reheat it directly, or it will split.