Thanks Jim. Actually, I did use Rice Krispies :-) Perhaps I shouldn't have called it 'puffed rice' but instead called it 'crisped rice', I wasn't 100% sure how to call it in English. Either way, you cannot use crisped rice or feuilletine in water based fillings (e.g. ganaches) because the water will make it soggy. However, they retain their crispiness when you use an oil-based filling (e.g. gianduja, hazelnut praliné, ...). The one I made was oil-based so the crisped rice worked fine. If you do want to use a ganache and also get crispyness, you could try a double layer approach: first a bottom layer with e.g. hazelnut praliné & milk chocolade & something cripsy, followed by a 2nd layer with a ganache. The moisture from the ganache won't migrate into the oil-based filling. The same applies for popping candy; works great in oil-based fillings, doesn't work at all in water based fillings.