It's easier to make this sauce if you make the reduction several hours before you're ready to make the sauce. I do mine the day before, along with clarifying the butter.

Place all the reduction ingredients in a small sauce pan and bring to a boil. Turn the heat down to medium and continue cooking, uncovered, until the liquid is reduced by half, about 20 minutes. Strain and reserve the liquid (should be 1/4 cup) to use in the sauce. The minced shallots/onion can be discarded, or dehydrate and save for seasoning in another dish.

The sauce should be prepared right before you're going to use it, otherwise it might break and not look as appealing (will still taste fine if broken, however).

In the bottom of a double boiler, heat water to not quite boiling then turn your heat down to low. Place the top section of the double boiler on it. Add the reduction liquid, which should be at room temperature. Constantly beating with a wire whisk or hand mixer on medium-low speed, slowly add the egg yolks to thicken. Still beating, very slowly add the butter, about 1 tablespoon at a time. When the butter is fully incorporated, immediately serve the sauce.