adapted from Mastering the Art of French Cooking
1 stick of butter, melted and set aside
3 egg yolks
1 Tbsp cold water
1 Tbsp lemon juice
Big pinch of salt
2 Tbsp cold butter, separated
Salt and freshly ground pepper (white, if you have it)
Few extra drops of lemon juice, if needed
Dash of Tabasco (optional)
Fill the bottom of a double boiler with a few inches of water. Bring to a bare simmer, then back the heat down.
While the water is heating, whisk the egg yolks in the upper part of the double boiler until thickened, about 1 minutes. Add the water, lemon juice, and salt, and beat for another 30 seconds. Add 1 tablespoon of the cold butter and place the pan over the water. Start whisking and don’t stop. Keep a pan of cold water nearby in case things start cooking too quickly. If the eggs begin thickening too quickly or suggest lumpiness, plunge the bottom of the pan into the cold water, continually whisking. Once the danger has passed, place the pan back over the simmering water.
Once the eggs have thickened to the point where you can begin to see the bottom of the pan between strokes, and the mixture forms a light cream on the wires of the whisk.
Immediately remove from the heat and beat in the other 1 tablespoon of cold butter.
Secure the pan on a dampened dishtowel. With the whisk in one hand and the vessel of melted butter in the other, start whisking the egg mixture and sloooowly drizzle in the melted butter by droplets. Once the sauce has thickened into a heavy cream consistency, you may pour the butter a little more rapidly. Do not pour in the milk solids in the dregs of the butter.
Taste and season the sauce with salt, pepper, lemon juice, and Tabasco. Pour, serve, and eat immediately.