Preheat the oven to 350°F (175°C) and cover a baking pan with parchment paper and grease with unsalted butter.
Melt the chocolate au bain-marie by adding the 6 oz chopped (or chips) bittersweet chocolate and butter in a heat-proof bowl over simmering water. Stir frequently. Once the chocolate is melted and smooth, remove the bowl and clean the bottom of the bowl.
Gradually add in the sugar while whisking. Then add in the eggs one by one making sure each one is well incorporated first. Add the cacao powder and mix. Add in the flour, salt, and mix well.
Pour half of the brownie batter into the baking sheet. Bake for about 10 minutes. Just as it starts to firm up.
After 10 minutes, take it out and spread the remaining chocolate chips on top. Pour the condensed milk over it and then pour the remaining batter. Use a wooden stick to help you spread the brownie batter and swirl it with condensed milk (especially the edges).
Bake the brownie for about 25 more minutes or until you insert a toothpick and it comes out with some dry crumbs (see note 1). Let them cool completely before cutting.