Preheat the oven to 350°F (175°C) and line an 8×8 or 9×9-inch pan with parchment paper.
Melt the butter until hot, then immediately stir in the cocoa powder to bloom it.
Add the sugar and whisk until the mixture is smooth and glossy.
Whisk in the eggs, vanilla extract, and salt until fully combined.
Gently fold in the flour just until it disappears.
Fold in the chopped chocolate and any optional mix-ins.
Bake for 35–40 minutes, until the edges are set and a toothpick inserted in the center comes out with moist crumbs.