Why is cocoa powder sometimes used in chocolate cakes when chopped semisweet chocolate is already used? I found a flourless chocolate cake   recipe that I want to try, it uses semisweet chocolate which I thought would have enough cocoa solids already but it calls for  cocoa powder, why is this?
You can't add too much chocolate into a cake, as you'll end up adding too much fat, which will change how the cake sets up.  They likely wanted more chocolate flavor than what they felt they could get from chocolate alone, and so opted to add cocoa powder as well.