Combine yolks, lemon juice and salt in blender. Blend for 10 seconds to combine.
Put melted butter in a measuring cup and slowly drizzle in with blender running. Together the blender and the hot butter cook the egg making a silky, smooth hollandaise sauce. Use sauce as soon as possible.
The hollandaise can be kept for about an hour. Keep covered in a warm spot near the stove or even in a Mason jar set in a small pot with some warm water in the bottom.
Notes
OPTIONAL – Many cooks like a pinch of cayenne in their hollandaise for a bit of heat.