Preheat oven to 350℉ (175℃). Line a 9"x13" or an 8"×8" (for thicker brownies) baking pan with parchment paper. Then, grease with butter or non-stick baking spray and set the prepared pan aside.
Combine the chocolate chunks and butter in a medium microwave-safe bowl. Then melt them in the microwave, stopping and stirring frequently every 20-30 seconds so they don't scorch or burn. It usually takes about 2 minutes.
Stir the chocolate continuously until it's smooth, shiny, and completely melted. If the microwave isn't your thing, melt it in a double boiler over medium heat.
Whisk the eggs in a large bowl. Add sugar, brown sugar, vanilla extract, and salt, and continue whisking until combined.
Stir the melted chocolate and butter into the egg mixture. Then, gently fold in the all-purpose flour and unsweetened cocoa powder until thoroughly mixed.
Pour the brownie batter into the greased or parchment-paper-lined baking pan and spread it out evenly.
Bake it until a toothpick inserted into the center comes out clean (about 30 minutes).
Let it cool, slice, and serve it all on its own. Or add a glass of cold, refreshing milk and top it with whipped cream.