According to food journalist, author, and advanced cicerone Mandy Naglich, mustard's tartness can amplify the bitter, earthy cocoa flavors in brownies.
As opposed to basic yellow mustard, experiment with more complex mustards, such as a sharp dijon or spiced brown mustard, to complement the warm toastiness of cocoa.
You can also use sweet or gourmet mustards to bring even more nuanced flavor to your brownie mix. When first incorporating mustard, it's best to start with a basic brownie batter.
This way, you can better gauge flavors and adapt. Otherwise, recipes with simple additions like chopped walnuts, dark chocolate chunks, or flecks of sea salt can fare well, too.
Since mustard is quite potent, using no more than a dollop should do the trick. However, as always, you can adjust the amount based on your preference.