Example: Recipe calls for 1oz square unsweetened chocolate (aka dark chocolate).
Substitution: 1 square (1 oz) Unsweetened Chocolate = 3 Tablespoons cocoa powder unsweetened plus 1 Tablespoon of shortening (here in Scotland I use White Flora for this) or margarine (not butter).
So if you need 3 squares therefore triple the substitution above so you'll end up with 9 tablespoons of cocoa and 3 tablespoons of the fat above!
That has helped me for over 10 years since I've been here. Just last night I've made a chocolate custard for a cream pie, and I've used the above substitution. Can't wait till diner for that pie. If I can get a piece before the rest of my family jumps in it LOL!