It would appear, after searching a number of sources, that Spain's King Ferdinand VI was terribly smitten by his mistress Santa Barbara, and over the years had initiated the construction of a church in her honour. Or, depending on the interpretation, a church now stands in her honour.
We were walking in downtown Madrid last evening when we came across the church, recently renovated and cleaned to period, and it is beautiful. Look at the detail on the front of this building.