I've been making this since I first learned to cook - a long time ago! This is great over veggies - asparagus, cauliflower, broccoli in particular. And, of course, eggs - even if you don't go in for the full production of Eggs Benedict (which are quite nice even without the English muffin)

3 egg yolks
1-2 Tbls lemon juice
1/8 tsp Cayenne pepper (or to taste)
A pinch of salt if desired (I seldom bother) (leave out if using salted butter)
1/2 cup unsalted butter (1 stick), melted till bubbly

While butter is melting put the yolks, lemon juice, and Cayenne into the blender, and blend on low till thoroughly mixed. When the butter starts to bubble pour it very slowly into the blender, with the motor going; if you pour to fast the sauce the sauce won't emulsify. If needed the sauce can be poured into the top part of a double boiler, and kept warm over barely simmering water. Leftover sauce can be re-heated in the same way.