Step 1: Prepare your double boiler. Fill a small saucepan with 1-2 inches of water and bring it to a gentle simmer. Ensure your heatproof bowl (stainless steel or glass) fits snugly over the saucepan without touching the water.
Step 2: In the heatproof bowl, combine the egg yolks, 1 tablespoon of cold water, and a pinch of salt. Place the bowl over the simmering water.
Step 3: Whisk the egg yolk mixture vigorously and continuously. The yolks will lighten in color, thicken, and become foamy. Continue whisking until the mixture forms a thick ribbon when you lift the whisk, about 3-5 minutes. This pasteurizes the eggs and creates a stable base.
Step 4: Remove the bowl from the heat for a moment. Begin slowly drizzling the very hot melted butter into the egg yolk mixture, a few drops at a time, while continuously whisking with your other hand. This is crucial for emulsification.
Step 5: As the sauce begins to thicken and emulsify, you can gradually increase the butter stream to a thin, steady trickle, still whisking constantly. Continue until all the butter is incorporated and the sauce is thick, smooth, and glossy. If the bowl gets too hot, lift it off the heat briefly while continuing to whisk.
Step 6: Remove the bowl from the heat completely. Whisk in the fresh lemon juice and the pinch of cayenne pepper (if using). Taste the hollandaise and adjust seasoning with more salt or lemon juice as desired.
Step 7: Serve your homemade hollandaise sauce immediately. If not serving right away, keep it warm in its bowl over a very shallow bath of barely warm (not simmering) water, or in a preheated thermos for up to an hour. Avoid direct heat or prolonged holding.