wine sauce: trouble with emulisification

You need a stabilizing agent you can use heavy cream or egg yolks or a combination of the two and this will keep it together better while adding richness and velvety texture. If you boil the sauce with the butter using your recipe then that's exactly what did it. Cream and egg yolk is referred to as a liaison btw

I agree with Chef June. The most recent edition of Cook's Illustrated covers this very topic for both red & white wine sauces.

Thank you June!

You'll have better results if you reduce the wine fully first, then add the rest of the liquid and reduce again. Your shallots will come out better if you do them separately, then add them and their butter to the reduced sauce.

Well that does sound like that butter was the problem.Just to be extra careful next time, you might want to add the sautéed shallot AFTER the butter has been whisked in at the end. And you do want to reduce those elements separately. I have read explanations of why, but cannot remember them.

To clarify, I sweated 1 shallot in about 1 tb butter until translucent.
I added about 1/4c red wine and 2 tb balsamic & 1/4c chix stock.
Then I reduced to syrup. It looked pretty good.

Then I set aside to do rest of dish and that's when it separated.
I never added addt'l butter.

Thank you Chefs.
I also wonder if I had too much butter when I was sweating the shallots.

The secret is reducing the wind down to a syrup before adding any more liquid. If that first step doesn't occur, your sauce will be watery, and there's really nothing you can do to make it right -- satisfactorily. Of course, there's always the dreaded cornstarch, but that's never going to give you the consistency and texture you're looking for.

Adding to much butter or not reducing the wine far enough can cause this type of reaction. Any time you reduce a red wine sauce you reduce to Au sec (Almost dry) this way you lose most of the water content

Its so important to link a recipe- so we can best advise.
Now, firstly, and this has nothing to do with answering your question (sorry) but why do you put sugar in a wine reduction? ugh. And balsamic is very sweet.... I do a braised mushroom dish with red wine and balsamic, but i would not want to see that anywhere near meat or poultry. Americans are just toooo sweets addicted. Plse- i do not mean to insult, but I continue to feel depressed by seeing sugar and salt everywhere.....

Back to your question. It sounds likely that you did not reduce it enough before you added your butter, and then it got too hot while it sat-so that the butter separated out. You usually want to reduce each component separately and they have to go down to a syrup, before adding in and reducing the next component. When all is reduced, you should have a syrup still; then you taste and adjust flavors; and then you whisk in your room temp butter and serve.
Does that make sense- how I described the process?