Bonjour Andrea,

The waiter suggests a dessert of chestnut-flavoured ice cream. "La glace de marrons"

You seem to be proposing "Glace aux marrons" where we have the plural noun les marrons (pieces of chestnut) in an ice cream format?

This is quite different from the ice cream of (de marrons) chestnut flavour.

I  think the key here is the preposition (de) suggesting "of chestnut flavour" rather than your proposal.

Hope this helps to explain for you.

Bonne continuation,

Jim

À propos « glace de marron » : Et pourtant on dit « la mousse au chocolat » . Il n’y a pas de morceaux du chocolat. C’est juste le goût de chocolat. Alors, pourquoi on dit « glace de marron »?

The difference is the same as in English:

une glace de marrons -- an ice cream made from chestnuts (the main ingredient is chestnuts)
une glace aux marrons -- an ice cream made with chestnuts (chestnuts are not the main ingredient)

Andrea and Nancy, 

As a keen cook I searched many French recipe sites on this, and did a google search. 

I couldn’t find any reference across numerous French cooking and recipe sites to ‘glace de marrons’  - ‘glace aux marrons’ or ‘glace à la crème de marron(s)’, but no reference to ‘glace de marron(s)’ showed up in my search. Not surprising - if it is ‘une vraie glace’.

My wife has never heard it referred to as just ‘glace de marron(s)’ either.

It is apparently not standard usage.