3 large egg yolks
1 Tbsp. lemon juice
1 Tbsp. hot water
2 sticks (4 oz each) unsalted butter
Pinch of cayenne pepper
Pinch Kosher salt
Add the egg yolks, lemon juice and hot water to the blender cup (or a cup that will just barely hold the head of your blender).
Melt the butter in a small saucepan over medium-low heat until the butter registers 180 to 190 degrees on an instant-read thermometer. Transfer it to a liquid measuring cup, leaving behind the thin layer of whitish liquid (discard it).
Insert the head of blender into the bottom of the cup and run the blender. Slowly pour in the hot butter. You should see the sauce begin to form at the bottom of the cup. As the sauce forms, slowly pull the head of the blender up to incorporate more melted butter, until all of the butter is incorporated and the sauce has the consistency of heavy cream. Season with the cayenne and salt to taste. Cover, and keep in a warm spot (not over heat) until ready to serve.
Note: Cooled hollandaise can be very carefully reheated over the lowest-possible heat while whisking constantly. Hollandaise can't be refrigerated and then reheated.