Put the egg yolks, lemon juice, salt and cayenne in a blender and buzz it for about 30 seconds on medium power.
Drop the speed of the blender to low and take the insert out of the top of the blender. Slowly drizzle in the melted duck fat and butter. Return the insert to the top of the blender, turn the power up to high and buzz for 15 seconds or so.
Serve at once, although Hollandaise can be held for up to a few hours if you keep it in a warm place, like on top of your stove if you are cooking.