Prepare your tin: Line a 9x13 inch baking dish with a long sheet of aluminium foil, leaving some overhang on two sides. This will act as handles later. Press the foil neatly into the corners—you'll thank yourself later when lifting out the set fudge.

Melt the base: Place the chopped dark chocolate, sweetened condensed milk, and cubed butter into a medium-sized heatproof bowl. Set it over a pan of gently simmering water—make sure the bottom of the bowl doesn't touch the water. Stir slowly and steadily with a rubber spatula. As the chocolate begins to melt, you'll notice the mixture turning glossy and smelling deeply of cocoa and butter.

Add the peanut butter: Once the chocolate is almost completely melted and the mixture is smooth (you'll see it become thick and luscious, like a warm ganache), add the peanut butter and vanilla extract. Continue stirring. The colour will shift to a warm, caramel-like brown, and the aroma will become nutty and inviting. Stir until every streak of peanut butter has disappeared.

Season and texture: Remove the bowl from the heat. Stir in the sea salt flakes and, if you're using them, the chopped roasted peanuts. You should hear a soft scrape of salt against the bowl, and the peanuts will give the mixture a pleasant, hearty look.

Pour and spread: Tip the mixture into your prepared tin. Use the spatula to coax it into every corner, smoothing the top as evenly as you can. It will be thick and slightly resistant, but keep at it. The surface should look uniformly dark and glossy.

Create the swirl: In a small, separate bowl, melt the milk chocolate chips in the microwave in 20-second bursts, stirring between each, until smooth. Drizzle this melted milk chocolate over the fudge in zigzag lines. Take a skewer or the tip of a knife and drag it through the lines to create a marbled effect. The contrast of the lighter milk chocolate against the dark fudge is beautiful.

Set and slice: Place the tin in the refrigerator, uncovered, for at least 2 hours, or until the fudge feels firm to the touch. Once set, lift it out using the foil overhang. Peel back the foil, place the slab on a chopping board, and use a sharp knife to cut it into squares. The knife should glide through cleanly, revealing a smooth, dense interior.