This nearly foolproof technique lets the blender do the sauce-making for you, yielding a slightly thicker hollandaise that is still creamy, smooth and perfect for topping poached eggs or steamed asparagus.
Melt butter in a saucepan over low heat and let cool slightly.
Add egg yolks to the jar of a blender. While the motor is running, slowly pour in the melted butter. When all of the butter has been added, season with lemon juice, salt, cayenne, and black pepper. Blend to combine. The sauce should be thick but still able to drizzle from a spoon (and it should form a pool, not a mound). If it is too thick, thin it with a little water.
The sauce is best if used immediately but can sit for about 30 minutes over hot water in a bain-marie or in a Thermos.