I completely LOVE this song, I even searched for the full version and downloaded it.
It matches so good the story of Bihter and Behlul, the lyrics are great
I don't know if it was written before, or especially for the series.
I love the "yasak ask" too. The violins in the beginning are sooo sad.
This music is my phone's ringtone right now
In case you don't know, his name is Toygar Işıklı. He composes a lot of the music for tv series. Ask-i memnu, yaprak dokumu and our aFatmagulun sucu ne? All his songs are wonderful.