Hollandaise sauce is really easy to make with 6 simple ingredients - egg yolks, lemon juice, melted butter, and some flavorful additionals such as Dijon mustard, salt, and cayenne pepper.
To the immersion blender add all ingredients, except the butter. Blend those ingredients.
With immersion blender on, slowly add melted butter, blending as you go. Blend until pale and thick.
Notes
Some recipes state that the melted butter needs to be hot. Chef Antoine says the trick is not the hot butter, but rather drizzling the butter in slowly as you blend.
Homemade hollandaise sauce is best served right away, however, you can store it in an air-tight container kept in the refrigerator for 2-3 days if needed, any longer than that and you risk the sauce separating.