Treacle tart

A women who appears to be a sweethearted, trustworthy, caring individual in public to people outside her circle of friends, relatives as well as relationships, including co-workers, but is actually supercilious in nature casting harsh and content insults often in fits of selfish rage. Based on a tart made from black treacle, a darker molasses, that has a distinctively strong, slightly bitter flavor that is disgusting if sampled from the jar.

"My sister is a total treacle tart." warned Joe to his friend Dan when his sister started to hit on him.