Preheat oven to 350°F and line the 9x13 inch pan with parchment paper.
In a bowl, whisk together flour, cocoa powder, salt, and baking powder.
In a separate bowl, cream together butter, granulated sugar, and brown sugar until fluffy.
Add eggs one at a time, mixing well after each addition.
Stir in vanilla and red food coloring.
Add buttermilk and vinegar, mix until combined.
Gently fold in the dry ingredients, being careful not to overmix.
Pour into prepared pan and bake for 25–30 minutes, or until a toothpick comes out with a few moist crumbs.
Cool completely on a wire rack for 1 hour.
Frost with cream cheese topping if desired before serving.
Notes
Use gel food coloring for best red color. Cream cheese swirl or frosting adds a nice tangy contrast to the fudgy base. Optional but highly recommended!