When I lived in NJ and CT, these sodas could be gotten at just about any soda fountain. Now that I live in OH, I got sick of trying to explain how to make these (here, a "black & white" is a cookie with vanilla and chocolate icing on it), and got my own ingredients.
Place chocolate sauce and milk in the glass you plan to use (at least 12 oz. capacity) and stir well. Pour seltzer water a little less than half full, stir gently. Add ice cream, one scoop at a time. being careful not to overflow glass; stir again (gently). Add more seltzer, if necessary, and top with Redi-Whip if you like.
Feel free to play around with the amounts of chocolate and milk.
A lower-fat version can be made by simply using skim milk and low-fat ice cream. Go ahead and use the Redi-Whip, anyway.