How did the dessert Napolean get its name?

You will find many references attributing the dessert NAPOLEON to Napolean Bonaparte. Don't take this for granted, it could be a corruption of Neapolitana meaning 'from Naples'. You will have to search through a lot of tedia on the following website, but in it there is a well-argued speculative view supporting the latter theory:

http://www.foodtimeline.org/foodpies.html#napoleons
https://en.wikipedia.org/wiki/Mille-feuille#Variant_names_and_forms

Napoleon pastry (nuh-POH-lee-uhn) - It is known as mille-feuilles in France. Outside of France it is known as 'Napoleon.' It consists of layers of puff pastry interspersed with pastry cream or whipped cream and iced with fondant and chocolate or with confectioner's sugar.
History: (1) It is believed to have been developed in France during the latter part of the 19th century. The Danish people have been told for generations that a Danish royal pastry chef invented the dessert way back in the 1800s on the occasion of a state visit between the Emperor Napoleon and the King of Denmark, in Copenhagen. Some sources believe that the chocolate lines on the pastry appear to form the letter 'N' for Napoleon. (2) A final story or tale is that the dessert was really a French invention after all, and that it was Napoleon's favorite pastry. It is said that he ate so many of them on the the eve of Waterloo that he lost the battle.

I'm with Barrowboy on this one -

Napoleons
The general concensus among the food history books is that 'napoleons,' a popular flaky pastry dessert, were not named for the famous emperor.
'Napoleons...have nothing to do with Bonaparte, the daring Corsican...The name is the result of a misunderstanding of the French word 'Napolitain' which should have been translated as 'Neopolitan' pertaining to Naples. They are very much like the French 'mille-fueille' or the Italian 'mille foglie' both of which mean 'a thousand leaves'.'
---Rare Bits: Unusual Origins of Popular Recipes, Patricial Bunning Stevens (p.202).