You can certainly omit it, but I highly recommend to include it to help bring out that rich chocolate taste of the loaf. (lemonandmocha.com)
As I took my first bite in to one of these brownies I got a very rich chocolate taste and then a good crunch from the walnuts. (laurasglutenfreekitchen.com)
They turned out fantastic and have a really nice rich chocolate taste. (smittenkitchen.com)