I realize that this reply is almost 2 years late; however, this post comes up as the #2 result in Google when I searched for "whole foods date walnut cake" so I feel justified.
After many searches the name of the thing is Pan De Dátil (dates & walnuts), and it is related to the seemingly more popular Pan De Higo (Figs and Almonds). I couldn't find a recipe for the date cake anywhere.
I found two recipes for Pan De Higo that are basically the same ingredients: figs, almonds, spices (cloves, anise seeds, sesame seeds, cinnamon) and honey. Maybe the same preparation steps in these recipes could be used with dates and walnuts, and just omit the spices and honey?