here, though, the brownies are merely soft, rather than pleasingly jaw-exercising, right? I'm trying to figure out how to explain the use of chewy that signals approbation and it's not an easy task directly, because it involves a particular kind of yielding resistance. Perhaps it's easier to go with a metaphorical usage: you can talk about a good, chewy argument or paper - something you can chew over, get your teeth into.