A good exercise for building mass on the triceps would be the California Press.
The California Press is a hybrid movement, that is a cross between a Close-Grip Bench Press and a Lying Triceps Extension.
It is a very popular assistance movement among powerlifters, particularly those who need to increase their triceps mass and strength to improve their bench press performance.
Assume the same starting position as for a Close-Grip Bench Press. Lower the barbell to your upper chest by bringing your elbows down and forward as you lower the bar. As you complete the range of motion, you will experience a great stretch in your triceps.
In the bottom position, your forearms should be in contact with your biceps, and the bar should be in contact with your upper chest. From this position, reverse the movement, pushing the bar upward from your chest. Extend your elbows to just short of lock-out in order to maintain tension on your triceps.
Here is coach Benjamin Siong of the Australian Strength Performance demonstrating the California Press
A good sensible starting weight would be somewhere halfway between what you use in the lying triceps extensions and the close grip bench press
Thanks to coach Benjamin Siong of the Australian Strength Performance Center in Melbourne Australia for the video