Reduce 1 cup (250 milliliters) fish stock (which includes the white wine) by two-thirds. Let the reduction cool and combine it with 5 egg yolks. Whisk the mixture over the heat until it becomes airy and thickens. Finish with butter in the same way as preparing Sauce Hollandaise. Classic recipes call for almost 4 ounces (125 grams) butter per egg yolk, but the sauce can be made lighter by using half as much, depending on the amount of butter used