Following on from my softening butter thread earlier, cupcakes are now done, but when i took them out of the baking tray, they all left a residue of butter behind which must have seeped through the case, and the bottom of each case was all buttery.
I can't taste one because I've made a dozen and I need a dozen for dd's nursery class, and I'm not that worried - they will probably just lick the icing off anyway, as they are 3, but I think that if they have lost butter in the baking process, they might have an odd texture, and I'd like to avoid doing it again.
They'll taste fine, and probably won't even feel as greasy by the time the children get at them.
Different fats have different effects on sponge mixes. It's been ages since I baked with butter (we're dairy-free ATM), IIRC butter makes sponges slightly greasy with a larger crumb, but they taste better and keep better than if they were baked with marge.