Preheat the oven to 325℉/160℃/Gas Mark 3 and line a 9x9 baking dish with parchment paper.
In a medium bowl, whisk together the granulated sugar, all-purpose flour, cocoa powder, icing sugar, salt, and dark chocolate chips.
In a separate large bowl, whisk together the melted butter, milk, vanilla extract, and large eggs.
Slowly pour the dry ingredients into the wet ingredients a little bit at a time and stir with a silicon spatula. The brownie batter will be thick.
Pour the brownie batter into the lined baking dish and spread it out as evenly as possible. Top with the extra dark chocolate chips here.
Bake in the oven for 50 minutes to an hour or until a toothpick inserted comes out with only a few crumbs on it. Do not overbake. If they seem a bit gooey in the middle that will be fine as they will set with time.
Remove from the oven and let cool for at least an hour or two before serving. The longer that they cool for the more set that they will become.