Melt butter in a small saucepan over medium-low heat until foamy and just starting to turn golden, about 3-4 minutes. Do not let it brown. Keep warm on lowest heat.

Fill a medium saucepan with 1 inch of water and bring to a bare simmer over medium heat. Reduce to low. Place a heatproof bowl over the pot, ensuring the bottom does not touch the water.

Add egg yolks and water to the bowl. Whisk vigorously until mixture is pale yellow, thick, and has doubled in volume, about 2-3 minutes. The whisk should leave visible trails. If mixture gets too hot and starts to scramble, immediately lift bowl off heat while continuing to whisk.

Remove bowl from heat. Whisk in melted butter a few drops at a time at first, then in a thin steady stream as the sauce thickens. Add lemon juice halfway through. The sauce should coat the back of a spoon and hold a line when you run your finger through it.

Whisk in salt and cayenne. Taste and adjust with more lemon juice or salt as needed. Serve immediately while warm, or hold for up to 30 minutes in a thermos or over the warm water bath off heat, whisking occasionally.