I only about 1/3 cup of dark cocoa powder. Any ideas to make up the difference? I have hot cocoa mix, chocolate chips, unsweetened chocolate squar...

Both the hot cocoa mix and the chocolate chips have sugar already in them, and may make the eventual cake too sweet.
I have found best results with unsweetened chocolate.
For every 3 tbsp of cocoa you need to replace, use 1 oz of unsweet chocolate and reduce fat in recipe by 1 tbsp (as the chocolate already has some fat).
Second choice is the chocolate chips: use 2 oz for every 3 tbsp cocoa powder; using these I would reduce both sugar & fat by 1 tbsp for every 2 oz of chips you use.