This is the Nişancı Mehmet Paşa mosque, according to the Strolling Through Istanbul guide NOT by Sinan, but some unknown architect. It dates from 1584-88 and was built for the Keeper of the Seal (Nişancı) Mehmet Paşa.

I came across it a couple of years back and when I am near always visit it, because it is a fine building, sitting in a nice garden in a curious neighbourhood.

The Strolling through Istanbul guidebook mentions that the Nisancı Mehmet Paşa mosque shows an interesting variation of the octagon inscribed in a square. Eight partly-engaged columns support the dome arches; in the axes there are four semidomes, while in the diagonals four smaller semidomes serve as squinches instead of penditives. The eastern semidome covers a projecting apse for the mihrab, while those to north and south also cover projections from the square. Also in the corners of the east wall there are two charming little kürsüs or platforms, access to which is gained by staircases built into the thickness of the wall from the window recesses. The choice of different types of marble is also praised. As a whole “the mosque is a masterpiece, it is as if the unknown architect, in the extreme old age of Sinan, had decided to play variations on themes invented by Sinan himself and to show that he could do as well as the master.”