Prep the pan: Line an 8x8-inch (20x20 cm) baking pan with parchment paper, leaving overhang on two sides for easy lifting. Lightly butter the parchment if you want ultra-smooth edges.

Set up your melting method: Use a heatproof bowl set over a pot with 1 inch of barely simmering water (double boiler). Make sure the bowl doesn’t touch the water. You can also use short bursts in the microwave at 50% power.

Add the base: To the bowl, add the chopped dark chocolate, sweetened condensed milk, and butter. Stir gently and continuously until the chocolate is almost fully melted and the mixture looks glossy and thick.

Finish the mixture: Remove from heat. Stir in the vanilla and sea salt until fully combined. The mixture should be smooth and pourable but thick.

Pour and level: Scrape the fudge into the prepared pan. Smooth the top with a spatula. If using toppings, sprinkle them on now and gently press to help them adhere.

Set the fudge: Let it cool at room temperature for 30 minutes, then transfer to the refrigerator for 2–3 hours, or until firm enough to slice.

Slice cleanly: Lift the fudge out using the parchment. Warm a sharp knife under hot water, wipe it dry, and cut into 1-inch squares. Wipe and warm the knife between cuts for clean edges.