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1
Bring the 30 ml of water to a full boil in a heavy saucepan. This establishes the aqueous phase without which the emulsion cannot form. The water must be actively boiling before any butter is added.
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2
Reduce the heat to the lowest possible setting. Add the cold butter one cube at a time, whisking continuously. Each cube must be fully incorporated before the next is added; adding butter faster than the emulsion can absorb it will break it. This step is where the dish lives or dies: the temperature must remain between 68 °C and 85 °C throughout — below 68 °C the emulsion solidifies and cannot absorb further butter; above 85 °C the milk-solid emulsifiers denature and the fat separates irrecoverably.
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3
Continue whisking in butter, cube by cube, until all 500 g are incorporated. Monitor temperature with a probe thermometer at every stage. The finished beurre monté should appear as a thick, opaque, cream-coloured liquid with a faint golden cast — it must not appear greasy or translucent, which would indicate a broken emulsion.
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4
Hold the finished beurre monté in a covered pot over the lowest possible heat or in a bain-marie, maintaining 68–85 °C. To poach lobster tail, bring the beurre monté to exactly 62 °C, submerge the tail, and hold for 15 minutes. The lobster is done when the flesh is just opaque throughout and yields to gentle pressure without resistance.