Preheat the oven to 350°F (180°C). Grease an 8x8 inch square pan with a nonstick oil spray or butter.
Then, line it with parchment paper and keep the parchment paper hanging over both sides. Set it aside.
Whisk flour, cornstarch, and salt together in a medium bowl. Set it aside.
In a bowl, combine milk and vinegar, stir, and let it rest.
Melt the butter in a saucepan or microwave and transfer to a large mixing bowl. Add the sugars to melted butter and stir until well combined, for about a minute.
Now add the milk + vinegar mixture to the butter mixture. Whisk well to incorporate and microwave this mixture for a minute. Remove the bowl from the microwave and whisk well until the sugar is completely dissolved and the mixture is smooth.
To check, rub a little mixture between your first finger and thumb. If it feels smooth without any grain, the mixture is ready. But if you find some grains, microwave the mixture again and stir well until the sugar is dissolved.
Add cocoa powder and vanilla, and stir well for a minute or until cocoa powder is fully incorporated.
Then add the flour mixture and chocolate chips and stir until just combined. Transfer the batter to the prepared baking pan and spread it evenly. Top it with more chocolate chips or chunks if desired.
Bake the brownies for 32-35 minutes or until a toothpick inserted in the center comes out almost clean.
Let it cool completely in the pan and remove gently by lifting the edges of the parchment paper. Then, slice the brownies into desired-sized squares.