This chocolate quick bread is enhanced with sour cream for a velvety soft texture. I put "no substitutes" next to "unsweetened chocolate because I think that is important to the overall composition, but it's a free country ;). If you do have to substitute, using 2 tablespoons of oil and 6 tablespoons of unsweetened cocoa powder might work. Still, this is really good with unsweetened chocolate.
For the flour I've used both White Lily and regular bleached Gold Medal which are both lower in protein than my usual unbleached King Arthur. I think the acidity in the bleached flour also affects the overall texture, and in this case in a positive way. I'll have to test this with unbleached to be sure. I also want to try making it with cake flour.