Preheat your oven to 350°F (175°C) and line an 8-inch square baking pan with parchment paper.

In a medium saucepan over low heat, melt the unsalted butter, then stir in the granulated and light brown sugars until combined.

Whisk in the eggs and vanilla extract until the mixture is smooth and glossy.

Pour in the vegetable oil and sift in the cocoa powder, stirring until fully integrated.

In a separate bowl, whisk together the flour, cornstarch, and salt, then fold into the wet batter until just combined.

Mix in the vinegar gently and pour the batter into the prepared pan.

Bake for 20-25 minutes or until a toothpick inserted comes out with a few moist crumbs.

Allow the brownies to cool completely in the pan on a wire rack.

For the frosting, beat the cream cheese and butter until creamy, then gradually sift in the powdered sugar and mix until fluffy.

Spread the cream cheese frosting over the cooled brownies, then cut into squares to serve.