Preheat the oven to 350°F (180°C). Grease a 9x9-inch square pan with nonstick oil spray or butter.
Line the pan with parchment paper, keeping the parchment paper hanging over both sides. Set it aside.
Whisk flour, baking powder, baking soda, and salt in a medium bowl. Set it aside.
Melt the butter in a saucepan or microwave and transfer to a large mixing bowl. Add the sugar and stir until well combined for about a minute.
Add buttermilk and whisk well until sugar is completely dissolved. If the sugar does not dissolve, microwave the butter mixture for a few seconds. Remove the bowl from the microwave and whisk well until the sugar is 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.
Add cocoa powder and vanilla, and stir well for a minute or until cocoa powder is fully incorporated.
Then, add the flour mixture and stir until it is just combined. Do not overmix. Add the chocolate chips and mix gently to incorporate them into the batter.
Transfer the batter to the prepared baking pan and spread it evenly. If desired, top it with more chocolate chips.
Bake the brownies for 25-28 minutes or until a toothpick inserted in the center comes out clean.
Let the brownie cool completely in the pan and gently remove it by lifting the edges of the parchment paper. Then, slice the brownies into the desired size of squares.