Specious means either misleadingly attractive or superficially plausible, but actually wrong.
Custard made with salt instead of sugar would add speciosity to your trifle. An example of speciousness is the specious claim that as chocolate is dervived from plants, a family sized bar of Aero counts as one of your five a day. That's only true if you choose the orange flavoured one (she adds speciously*)
*mint's a plant, so the green ones are OK too.