This is about as easy as it gets!
3 egg yolks
1 tablespoon lemon juice
1/2 teaspoon salt
a pinch of cayenne
8 ounces unsalted butter, melted
Place yolks, salt, lemon juice and cayenne in a blender or a container for an immersion blender. Blend until the yolks have lightened in color. With the bender running, start adding the melted butter very slowly until it is all incorporated. Taste for salt and lemon and adjust seasoning, if necessary.