Line a 20x20cm (8x8 inch) baking dish with parchment paper, leaving some overhang for easy removal.
In a microwave-safe bowl, melt the butter in short bursts of 30 seconds, swirling between each until just melted and fragrant with a rich, caramel aroma.
Add the sifted cocoa powder, sweetened condensed milk, and vanilla extract to the melted butter. Whisk everything together until smooth, glossy, and well combined, about 2 minutes.
Gently fold in the chocolate chips until evenly distributed; they will soften and start to ooze when stirred, creating a luscious mixture.
Pour the mixture into the prepared dish and use a spatula to spread it out evenly, pressing down gently to ensure it’s compacted and smooth on top.
If desired, sprinkle chopped nuts or flaky sea salt flakes over the top for added texture and flavor.
Cover the dish with plastic wrap and refrigerate for at least 2 hours, or until the fudge is firm, shiny, and slightly crackled on the surface.
Once set, use the overhang to lift the fudge out of the dish. Place on a cutting board and cut into squares with a sharp knife warmed slightly with hot water for clean, neat slices.
Serve the fudge bars chilled, enjoying their rich, fudgy texture and glossy appearance. Store leftovers in an airtight container in the fridge for up to a week.
