Preheat your oven to 175°C (350°F). Line your baking pan with parchment paper, letting some overhang for easy removal.
Melt the butter in a microwave-safe bowl, then let it cool slightly until it’s just warm and fragrant.
In a large bowl, whisk together the sugar and sifted cocoa powder until they’re combined and slightly fragrant.
Pour the warm melted butter into the dry mixture, stirring until smooth and glossy.
Add the eggs one at a time, whisking vigorously after each to incorporate and create a shiny, thick batter.
Stir in the vanilla extract, then gently fold in the flour until just combined, being careful not to overmix.
Gently fold in your chosen mix-ins, spreading them evenly throughout the batter.
Pour the batter into your prepared pan, spreading it out evenly with a spatula and tapping the pan lightly to settle the batter.
Bake for 20-25 minutes, until the edges look firm and slightly cracked on top, and a toothpick inserted in the center comes out with moist crumbs.
Once baked, remove from the oven and let cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.
After cooling, lift out the whole block of brownie using the parchment overhang, and slice into squares. Serve and enjoy the rich, fudgy texture with your personalized mix-ins.