How can I save a batch of truffles when I realized I only used 7 oz of chocolate instead of 9 oz and it doesn't seem to be thickening up enough?
Whether you are making the filling or the shell, this is recoverable.
If you are making the filling or center, just melt it down again, and add the additional chocolate. Let it cool and continue your recipe as normal.
If you are making the tempered shell, it is more difficult, but you can melt down, add the additional chocolate, and re-temper. This may not be worth it since you would need one of the more difficult "from-scratch" tempering methods. Still, you can use this chocolate for another filling, where tempering is not required.