Prep the pan. Line an 8x8-inch (20x20 cm) square pan with parchment, leaving overhang on two sides for easy lifting. Lightly mist with nonstick spray if you like.

Melt the base. Add the chocolate chips and sweetened condensed milk to a medium saucepan. Warm over low heat, stirring constantly with a spatula until the mixture is smooth and glossy. This takes 3–5 minutes. Don’t rush the heat.

Add peppermint. Remove from heat. Stir in 1/2 teaspoon peppermint extract. Taste a tiny dab and add up to another 1/4–1/2 teaspoon if you want a stronger mint flavor.

Pour and level. Scrape the mixture into the prepared pan. Smooth the top with an offset spatula. If using crushed candy canes or flaky salt, sprinkle them on now and gently press so they stick.

Chill to set. Refrigerate for at least 2 hours, or until firm enough to cut. For clean edges, chill 3–4 hours.

Slice and serve. Lift the fudge out by the parchment. Use a warm, sharp knife (run under hot water and wipe dry) to slice into 1-inch squares. Wipe the blade between cuts for neat pieces.