Sauce made from butter and cream flakes/curdles
Tender means soft and not brown. Sour cream will curdle if the fat content is not high enough (if it is high, it is called creme fraiche, techniquelly) and if you cook it down too much on too hot heat. Next time, use higher fat content and cook on lower heat. Cream, sour cream and creme freche are not really substitutions, not techniquelly and not taste-wise.Mixing butter and sour cream will work, if you take your time and boil down with hardly any or no bubbles.