I've been massively undercharging for my cakes since I've been in business (about 11 years). I recalculated my prices and I'd need to charge substantially more. Ex: $40 -> $78 for a 6" triple layer. What so I say to clients?

That  sure is a massive increase.  I suggest you do it in degrees.   Maybe 1/2 of that increase now and the other 1/2 in about 3 to 6 months.  What to tell them?   Just that they must be aware of the increases to all kinds - ingredients are only a fraction of the cost and  you have not been factoring in non edibles such as gas, electric, packaging etc.