Preheat the oven to 175 C (160 C fan). Butter the bottom and the sides of your 8" square baking tin and line it with baking paper, with plenty of overhang.
In a medium saucepan, combine the butter and 100g of sugar and heat over a medium heat, stirring slowly until the sugar has dissolved and the butter has melted completely, about 1-2 minutes. Set aside and allow to cool slightly.
In a large bowl, whisk together the eggs and the remaining 200g of sugar until light, smooth and fluffy. Pour the cooled sugar and butter mixture into the sugar and egg mixture and whisk to combine.
In a medium bowl mix together cocoa powder, salt, baking powder and the flour. Add the dry ingredients to the bowl with sugar, egg and butter mix and whisk to combine. Do not overmix, few streaks of flour remaining are just fine. Add the chocolate or chocolate chips and fold them with spatula until they are evenly incorporated. Pour the batter into the prepared baking tin and smooth the top with spatula. You can also reserve a small handful of chopped chocolate/ chocolate chips and scatter them over the top of brownie batter.
Bake the brownies for 28 minutes or until the top of the brownies is set, with few cracks in the surface. Toothpick inserted in the middle will not be clean and should have some crumb on it.
Allow the brownies to cool and set in the tin for at least 30 minutes or ideally, until completely cool. Use the overhang baking paper to lift the brownies from the tin, slice and enjoy!
Using significantly different size baking tin will require adjusting the ingredients and the baking time. If your baking tin is only slightly smaller, increase the baking time by a couple of minutes. If your baking tin is only slightly bigger, reduce the baking time by a couple of minutes.
Allow the brownies to cool completely before cutting them into slices.
Store sliced brownies in the airtight container at room temperature for 3-4 days. You can also store them in the airtight container in the fridge for up to 10 days.