Line your 9x9 inch pan with parchment paper, leaving some overhang for easy lifting later.
In a heavy-bottomed saucepan, melt the butter over low heat, listening for a gentle bubbling and smelling a nutty aroma after about 2 minutes.
Add the cocoa powder to the melted butter and stir continuously with a spatula until the mixture becomes smooth, glossy, and dark chocolate in color, about 1-2 minutes.
Pour in the sweetened condensed milk in one steady stream and stir constantly, watching the mixture thicken and become shiny, about 3-4 minutes.
Remove the pan from heat and stir in the vanilla extract along with a pinch of sea salt, integrating until fragrant and well mixed.
Pour the thick, glossy fudge mixture into your prepared pan and use a spatula to spread it evenly, smoothing the surface for a polished look.
Let the fudge cool at room temperature for about 20-30 minutes, then transfer it to the fridge to chill until firm, at least 2 hours or overnight for best results.
Once set, lift the fudge out of the pan using the parchment overhang and place it on a cutting board.
Dip a sharp knife into hot water, wipe dry, and slice the fudge into neat squares, observing the shiny surface and rich, dense interior.
Serve the fudge immediately or store leftovers in an airtight container in the fridge, where it will stay fresh for up to a week.
