Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper for easy removal.
In a medium saucepan over low heat, melt the butter and chopped dark chocolate together, stirring until smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, combine the granulated sugar, brown sugar, eggs, and vanilla extract. Whisk until well combined and slightly frothy.
Slowly pour the melted chocolate mixture into the sugar and egg mixture, stirring continuously to prevent the eggs from cooking.
Add the flour and salt to the mixture, folding gently until just combined. Be careful not to overmix.
Fold in the diced rhubarb and dark chocolate chips, if using, until evenly distributed.
Pour the batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 15 minutes before slicing into squares. For best results, chill in the refrigerator for an hour before serving.