Prep the pan: Heat oven to 350°F (175°C). Line an 8-inch (20 cm) square pan with parchment, leaving overhang for easy lifting.
Lightly grease the parchment.
Melt the chocolate and butter: In a heatproof bowl set over barely simmering water, melt milk chocolate and butter, stirring until smooth. Remove from heat and let cool 5 minutes. (Microwave in 20–30 second bursts if you prefer, stirring between each.)
Whisk the sugars and eggs: In a separate bowl, whisk granulated sugar, brown sugar, and eggs for 1–2 minutes until slightly thick and lighter in color. This helps the shiny top.
Add vanilla and salt: Whisk in vanilla and salt until combined.
Combine with melted chocolate: Slowly stream the warm (not hot) chocolate-butter mixture into the egg mixture, whisking constantly until smooth.
Fold in cocoa: Sift cocoa over the bowl and gently fold until no dry spots remain.
The batter will be glossy and thick. Stir in mix-ins if using.
Pan and bake: Spread batter into the prepared pan and smooth the top. Bake 20–25 minutes, or until the edges are set and the center jiggles slightly with a toothpick showing moist crumbs, not wet batter.
Cool completely: Place the pan on a rack and cool at least 1 hour.
Lift out using the parchment and slice with a warm knife, wiping between cuts for clean edges.