Preheat your oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper, leaving an overhang for easy removal. Lightly grease the paper.
In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until combined.
Whisk in the eggs one at a time until fully incorporated. Stir in the vanilla extract and white vinegar.
In a separate bowl, whisk together the flour, cocoa powder, salt, and baking powder.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
In a small bowl, mix the buttermilk and red food coloring until you achieve a deep red color. Gently fold the red buttermilk mixture into the batter until streaks disappear.
Pour the batter into the prepared pan and spread evenly.
Bake for 25 to 30 minutes. The edges should look set, and a toothpick inserted near the center should come out with moist crumbs attached for a fudgy texture.
Let the brownies cool completely in the pan on a wire rack.
Prepare the frosting: In a medium bowl, beat the softened cream cheese and butter until smooth. Gradually beat in the powdered sugar until fully combined. Add the vanilla extract and milk/cream, beating until the frosting is light and fluffy.
Once the brownies are completely cool, spread the cream cheese frosting evenly over the top.
Cut into squares and serve.
Notes
For an extra rich chocolate flavor, use high-quality cocoa powder.
If you prefer a cream cheese swirl instead of frosting, reserve half of the batter, mix the cream cheese mixture into the remaining half, then swirl the two batters together before baking.
Chill the brownies for 30 minutes after frosting for cleaner cuts.