How to develop a butterscotch flavor in a savory sauce
Most likely, brown sugar and butter is the base. Salted butterscotch is quite nice as a dessert; you could easily add spices as it cools, and potentially dilute it with oil, cream or wine; it doesn't tend to taste very sweet when salted, anyway, if the brown sugar is sufficiently caramelized.

Alternatively, you might just be encountering a brown butter, which is butter cooked until the milk solids slightly brown. Usually this is strained with a fine mesh to separate some of the solids, but not always. Salt and pepper can be added for flavor, not to mention other possibilities. I've often made brown butter by cooking it with fresh sage until the sage crisps, then removing the herbs as the butter finishes browning.