In a tall container, whisk the egg yolks with the lemon juice, lightly salt and pepper. Melt the butter. Mix up the egg mixture with the hand blender and slowly pour in the melted butter. Mix a little more. Do not swing the blender back and forth, but always pull it up and down!
If you like, you can mix in a tablespoon of crème fraîche. Season to taste with salt and pepper.