How long after “best before” date is it safe to use unopened Williams Sonoma veal Demi glacé.

"Best before" is not a safety date. It is used as a combined recommendation for best color, taste, quality and to move product off shelves (marketing technique).
Open it, smell it, taste a bit.
If questionable or unpalatable, toss it.
If ok, keep in fridge about 4 days, in freezer about 4 months.