Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined. Set aside.
In a separate bowl, mix the melted butter and brown sugar until smooth and creamy.
Add the egg and vanilla extract to the sugar mixture, blending well until fully incorporated.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix.
Fold in the chocolate chips and chopped walnuts if using.
Using a tablespoon, scoop the fudgy brownie batter onto the prepared baking sheet, leaving some space between each scoop.
Take the cookie dough and flatten it between your palms slightly, then place it atop or beside the brownie scoops, slightly pressing down to adhere.
Bake in the preheated oven for 12-15 minutes, or until the edges are set, but the centers are still chocolatey and slightly underbaked for that fudgy texture.
Once baked, remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Notes
Serve the brookies on a decorative plate, drizzled with melted chocolate and sprinkled with sea salt for an extra touch of indulgence.