How can I make 'rich chocolate' flavor?
Frankly, Hershey's cocoa powder is low quality. Buy a higher quality cocoa. One objective measure is the cocoa fat content. 

This is from Harold McGee's Keys to Good Cooking, published by The Penguin Press, New York, 2010, p. 476:


  Higher-fat cocoas make richer dishes. To compare the fat contents of different brands, check their nutrition labels.