Derby pie is a chocolate and walnut open-faced custard pie baked in a (pre-baked) pie shell. The pie was created in 1950 in the Melrose Inn of Prospect, Kentucky, United States, by George Kern with the help of his parents, Walter and Leaudra. It is often associated with the Kentucky Derby. The name "derby pie" was chosen because the various family members each had a different name for the creation, so to resolve the naming quandary they put the various names in a hat, and pulled out the paper which said "derby pie".

Source: Wikipedia