This is a mosque built by Nişancı Kara Davut Paşa toward the end of the 15th century. Strolling through Istanbul (Redhouse) describes it as unique in Istanbul, because it is a "broad, shallow room divided into three sections by arches, each having a dome". There was a service going on when I passed it and from an earlier visit I did not remember it as special, so just a picture of the entrance.