what is a substitute for shortening?

Same volume of oil, already suggested, or animal fat if you use it, though more often used in pastry than cakes.
Probably need 1.25x butter compared to the oil volume in original recipe because the butter is about 1/5 water.

This does sound delicious and decadent. Since the shortening is melted with a portion of the chocolate (rather than creamed with the sugar), you could use coconut oil, or probably even a vegetable oil in place of the shortening. You could also use butter.