I'm relatively new at this and have a request for a chocolate filling that is neither too sweet, nor too chocolatey. I've actually never had anyone ask for chocolate "but not too much", so I'm at a bit of a loss for what to use. My first thought was mousse, but I'm worried about it holding up under fondant and in our summer temps. Then I thought whipped ganache, but I'm thinking that might be too rich for my customer's taste. I know you guys can help me to find the right solution! FWIW, it'll be paired with a simple vanilla cake. Please feel free to link me to the appropriate thread if one already exists. Thanks in advance!

Hi,
I don't know if this is too late,
have you tried chocolate buttercream?
You could experiment with the amount of cocoa powder you put in so its not too chocolatey,
I have a recipe, I live in the UK so its a uk recipe,

chocolate buttercream icing
110g/4oz butter, softened
170g/6oz icing sugar
55g/2oz cocoa powder, sifted
1-2 tbsp milk

you could make the buttercream recipe, with the butter and icing sugar and add the cocoa powder bit by bit until its as chocolately that you think it should be.
Good Luck!

It's probably because chocolate cake can be so rich on it's own that the client is concerned that a sweet, chocolatey filling would be OTT.

I would suggest chocolate SMBC. Mine always gets rave reviews because it is light and not overly sweet.