Hi manyhats :-) It's so nice to see your post since we haven't heard from you in a lo-o-ong while:-) I found this recipe on the internet and have not tried it. I think that if the sauce seems too thick when you reheat it, that you can just add water to thin it some since water is one of the ingredients in the recipe. As I recall, that is what I have done in the past when I had Hollandaise Sauce become too thick but I have not made it in a long time. When DH makes Eggs Benedict, he uses a Cheese Sauce which, btw, is delicious with Eggs Benedict. I think I posted the recipe here a long time ago. Anyway, here is the recipe I found. I hope that this helps.

Cooked Hollandaise Sauce
Source: The American Egg Board

Makes: about 3/4 cup (double for use in above recipe)

3 egg yolks
1/4 cup water
2 tablespoons lemon juice
1/2 cup firm cold butter, cut into eighths
1/4 teaspoon salt, optional
1/8 teaspoon paprika
Dash ground red pepper

In small saucepan, beat together egg yolks, water and lemon juice. Cook over very low heat, stirring constantly, until yolk mixture bubbles at edges. Stir in butter, 1 piece at a time, until melted and sauce is thickened. Stir in seasonings. Remove from heat. Cover and chill if not using immediately.