From America's Test Kitchen Family Cookbook.
This is incredibly easy - I have no idea why people try to make this on a stovetop.
3 egg yolks
1 tbs fresh lemon juice
2 sticks of butter, melted
Hot water
1/8 tsp cayenne
Salt and pepper
Method:
Add the egg yolks and lemon juice to a blender, blend until smooth and frothy
Keeping the blender running, slowly drizzle in half the butter.
Add 2 teaspoons of hot water.
Slowly drizzle in the remaining butter.
Add cayenne and salt and pepper to taste.
If not sufficiently thick, slowly add more water whilst running the blender.