In a small saucepan melt butter. Keep on low heat while you blend the other ingredients.
In the jar of your blender, combine egg yolks, lemon juice, dijon, kosher salt, and cayenne, if using. Blend on high until fully mixed, about 10 seconds.
Turn the heat back up on the butter to make sure it is very hot. Transfer to a cup with a spout. While blender is running, slowly drizzle the butter through the opening in the top. Run on high for 15 to 30 seconds. You will actually hear when the hollandaise is done when the sound becomes deeper in tone.
Taste for salt and check for thickness. If it needs more salt, add a tiny pinch more. If you would like it thinner, drizzle in 1 to 2 tablespoons of hot water while you run the blender. If you want it thicker, run the blender 15 to 30 seconds longer.
Serve as soon as possible.
