Philly has the cheesesteak, Chicago has the deep dish, and (for me at least) San Francisco has the "It's It" ice cream sandwich. A San Francisco favorite since invented by George Whitney in 1928, it has remain virtually unchanged: two oatmeal cookies sandwiching vanilla, chocolate or mint chip ice cream, and then covered with dark chocolate. (There is supposedly a cappuccino version, but I have never seen it in the wild.)
I find the "It's It" rendition of the ice cream sandwich to be a near perfect ratio of ice cream to cookie, not too big and not too small. The doughiness and density of the oatmeal cookie contrasts well with the soft creaminess of the ice cream. And the chocolate adds a nice contrast on the outside.
Available at your neighborhood bodega all over San Francisco for about $2.