Prepare the pan: Line an 8×8 baking dish with parchment paper, leaving some overhang on the sides for easy removal.
Melt the chocolate: In a medium saucepan over low heat, melt the chocolate chips and butter together, stirring until smooth.
Add the condensed milk: Pour in the sweetened condensed milk and continue stirring. The mixture will thicken slightly as the cooler milk hits the warm chocolate. Keep the heat low (don’t turn it up) to avoid scorching. Mix until glossy and fully combined.
Pour and smooth: Spread the warm fudge into the pan and smooth the top with a spatula.
Add toppings (optional): Sprinkle on your favourite toppings — peppermint, flaky sea salt, nuts, sprinkles — and gently press them in.
Chill to set: Refrigerate for 2–3 hours, or until firm enough to slice.
Slice and enjoy: Lift the fudge from the pan using the parchment and cut into small squares.