substituting for chocolate chips
As can seen on that chart, cocoa powder does not substitute for chocolate. The powder had the chocolate flavor, but none of the cocoa butter, the fat. So while it may be possible to make a mouse with cocoa powder, the recipe is bound to be different in more ways than one.
I guessed that a low-fat mouse would use cocoa powder. Here is an example:
I have no idea whether it is good or not.