Prepare the pan: Line an 8x8-inch (20x20 cm) square pan with parchment, leaving overhang for easy lifting. Lightly butter the parchment if you want extra insurance.

Set up a gentle heat: Add the chocolate, sweetened condensed milk, and butter to a medium heavy-bottomed saucepan. Place over low heat.

Melt slowly: Stir constantly with a silicone spatula until the mixture is smooth and glossy, about 5–7 minutes. Keep the heat low so the chocolate doesn’t scorch.

Finish the base: Remove from heat. Stir in vanilla and salt until fully combined. Fold in any mix-ins quickly while the fudge is still warm.

Pan and smooth: Scrape the mixture into the lined pan. Smooth the top with an offset spatula or the back of a spoon.

Set properly: Let the pan rest at room temperature for 20–30 minutes, then transfer to the fridge. Chill until firm, about 2–3 hours.

Slice cleanly: Lift the fudge out using the parchment. Warm a sharp knife under hot water, dry it, and cut into 1-inch squares, wiping the blade between cuts.