I'm making DH's birthday cake and I need to stack three square cakes together, so I figure I'll merge them with cream. How do I get the cream to be that super thick hard cream that cake shops manage? I've bought thickened cream, do I add a stack of icing sugar and let it set a bit before I use it? I can't have it runny as I need to cover the outside of the cake with buttercream icing.

TIA