Preheat your oven to 350°F (175°C). Grease your 8x8 inch baking pan or line it with parchment paper for easy removal later.
In a medium saucepan over low heat, combine the butter and the 4 ounces of dark chocolate. Stir until melted and smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, whisk together the sugar, eggs, and vanilla extract until well combined. Gradually add in the melted chocolate mixture, followed by the sweetened condensed milk, mixing until smooth.
In another bowl, sift together the cocoa powder, flour, and sea salt. This helps to ensure there are no lumps and that the dry ingredients are evenly distributed.
Gradually add the dry ingredient mixture to the wet ingredients. Stir gently until just combined; be careful not to overmix.
Gently fold in the chopped dark chocolate or chocolate chips until evenly distributed throughout the batter.
Pour the brownie batter into the prepared baking pan, spreading it evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan for about 10 minutes. Then, transfer them to a wire rack to cool completely before cutting into squares.
