Preheat the oven 325 or 160c. Line an 8x8 baking tin with parchment paper or grease the pan thoroughly.
In a large mixing bowl, add the hot, melted butter and sugar (it needs to be hot to dissolve the sugar). Mix until thoroughly combined. There shouldn't be any butter floating on top of the sugar mixture. Keep stirring, it will all come together!
Next, add the cocoa + vanilla and mix thoroughly. Add the eggs, one at a time, stirring until the eggs are compeltley combined before adding the next one.
Once the batter is combined, add the flour, arrow root (or corn starch), baking powder + salt. Stir until there are no more visible clumps of flour, but do not overmix.
Fold in the chocolate pieces. White chips are my favorite, but milk, dark or a mix are all great too.
Scrape the batter into the lined or greased pan and bake for 20-30 minutes. Be sure to check the brownies afer 20 minutes. A toothpick should come out *relatively* clean (sometimes the chocolate chips will make the toothpick appear dirty when the brownies are already done).
Leave the brownies to cool for AT LEAST 1-2 hours before trying to slice them. They are always perfect if they cool overnight.