Old School Method
Set up a double boiler: Bring a medium pot filled with 1 ½” of water to a simmer. In a heatproof bowl (preferably stainless steel), add egg yolks and lemon juice. Place the bowl onto the pot, checking to make sure the water is not touching the bottom of the bowl.
Whisk vigorously until the yolks become thick and creamy, about 3 minutes. (To prevent scorching your hand, wear an oven mitt or hold the bowl with a small kitchen towel.)
Slowly add melted butter a few tablespoons at a time while continuously whisking. Be sure that each addition is incorporated into the egg mixture before adding more. Continue whisking until sauce thickens, about 2 minutes more.
Remove from heat and season with ½ teaspoon salt, cayenne, and additional lemon juice if desired.
Quickie Method
Add egg yolks and lemon juice to a blender. Pulse to combine.
Turn the blender to low and slowly add all the (warm) melted butter while it is running. Continue to blend until the sauce is thick.
Season with ½ teaspoon salt, cayenne, and additional lemon juice if desired.
