In your blender, combine egg yolks, lemon juice, salt, pepper, and mustard. Blend on high speed to combine, about 20 seconds.
Heat your butter either on stovetop or microwave until completely melted and hot. This next step is very important. Put your blender on low speed, and slowly pour in the hot butter a little at a time. Do not pour in too fast, otherwise your sauce will not come together. The sauce should start to come together as you slowly pour in the butter, turning a pale yellow and thickening. Continue to blend for a few seconds after all the butter is incorporated. Taste the sauce. If you want, you can adjust the lemon, cayenne, etc. Serve sauce while still warm.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.