Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly from the start.
Line your 9x9-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal later.
In a medium saucepan over low heat, melt the butter and the 4 ounces of dark chocolate together, stirring frequently until smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, whisk together the melted chocolate mixture, sugar, eggs, vanilla extract, and sweetened condensed milk until fully combined and creamy.
In a separate bowl, sift together the cocoa powder, flour, and sea salt to avoid lumps.
Gradually add the dry ingredients to the wet ingredients, using a rubber spatula to gently fold them together. Be careful not to overmix.
Fold in the chopped dark chocolate or chocolate chips to the batter, ensuring even distribution.
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.
Remove the brownies from the oven and let them cool in the pan on a wire rack for 10-15 minutes. Then lift out using parchment overhang and allow to cool completely before slicing.