Preheat to 350°F (175°C) and line an 8x8 pan with parchment, grease lightly.
Whisk together flour, cocoa powder, salt, and baking powder.
Combine melted butter, sugars, eggs, oil, vanilla, red coloring, and vinegar.
Gently fold dry ingredients into wet until just mixed.
Spread batter evenly in prepared pan; add cream cheese swirl if using.
Bake 25–30 minutes until a toothpick comes out with moist crumbs.
Let cool completely before cutting into squares.