Preheat the oven to 350°F (180°C). Line a 9×9 inch baking pan with parchment paper. In a medium bowl, whisk together the flour, cocoa powder, and salt.
In a separate bowl, whisk together the melted butter, sugar, and brown sugar until smooth. Add the eggs, vanilla, and apple cider vinegar and whisk until fully combined.
Transfer mixture into the prepared pan and bake for 30-35 minutes or until a toothpick inserted in the center of the pan comes out clean or with a few moist crumb. Allow the brownies to cool completely.
Meanwhile, prepare the frosting by creaming together the butter and cream cheese. Add the powdered sugar in 2-3 smaller additions and the vanilla extract, mixing well after each sugar addition.