I frequent Jars of Clay every time i'm in Phnom Penh. It is a nice little quaint cafe just around the corner from Russian Market. Jars was started as a social enterprise, with the aim of providing young cambodian women with skills to ensure a decent and sustainable livelihood. The ladies today run and manage the entire cafe. The cakes/ food are all made by them as well!
The sheperd's pie and lagsane are my lunch favorites. The portion is huge, and it comes with a generous serving of chips or fresh vege. The cakes are delicious as well. The ones to try are lemon slice, carrot cake, passion cake.. actually, just try everything if you stomach affords it.
You can expect to pay about $5 to $6 dollars for mains, $3 for a drink and $1.50-$2 for cakes. Can be a little pricey compared to other cafes, but worth the try when you are near Russian market for a shopping trip!