I let them cool & sit out on the baking sheet while I do other stuff for around 30 mins - 1 hour, then once I fill them, store them in an airtight container and pop them into the fridge. The Fillings I used were chocolate ganache & Mango Jam (Just in case you thought the filling mattered). I am thinking if it could be because I switched icing sugar brands, and if so, how would the corn starch content affect this? Would the starch help develop a more solid shell or is it the other way around? The Corn Starch content isn't indicated on the packaging of my icing sugar. The brand I previously used had 2% corn starch and 98% Sugar. Any Ideas?