Line your baking dish. Take a 9x13 inch baking dish and line it completely with a large sheet of aluminium foil, leaving an overhang on two opposite sides. Press the foil into the corners so there are no wrinkles. This foil sling will let you lift the entire block of fudge out later without any struggle.

Melt the base. Place a medium saucepan over low heat. Add the chopped dark chocolate, sweetened condensed milk, peanut butter, and cubed butter. Stir constantly with a silicone spatula or wooden spoon. You'll see the butter and chocolate begin to soften and streak together – keep stirring. After about 4 minutes, the mixture will turn glossy and completely uniform, with no visible streaks of butter or chocolate. It should smell deeply of roasted peanuts and cocoa. Remove from heat immediately.

Add the flavourings. Stir in the vanilla extract and the fine sea salt. The mixture will become slightly more fluid and aromatic. You'll notice the vanilla scent blooming as it hits the warm fudge. Fold in the chopped peanuts and milk chocolate chips if using, distributing them evenly. The fudge will feel thick and heavy on the spoon, like a dense ganache.

Press into the dish. Scrape every last bit of the fudge mixture into the prepared baking dish. Use a silicone spatula or the back of a spoon to spread it evenly into all four corners. The surface should be relatively level – don't worry about perfection, as the fudge will be cut into squares. Sprinkle the top with a pinch of flaky sea salt for that salty-sweet contrast.

Chill until firm. Place the dish in the refrigerator, uncovered, for at least 2 hours. The fudge needs this time to set completely. You'll know it's ready when the surface feels firm to the touch and the foil peels away cleanly from the edges. If you're in a hurry, you can pop it in the freezer for 45 minutes, but fridge setting gives a creamier texture.

Cut and serve. Once fully set, use the foil overhang to lift the entire block out of the dish. Place it on a cutting board. Use a sharp knife to cut into 24 even squares – I find a warm knife (run under hot water and dried) gives the cleanest cuts. The fudge should slice with a clean break, revealing a dense, fudgy interior studded with peanut pieces.