Prep the pan. Line an 8x8-inch (20x20 cm) baking pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment with butter to prevent sticking.

Set up gentle heat. Add the chocolate chips, sweetened condensed milk, and butter to a medium saucepan. Place over low heat. Stir slowly and constantly with a rubber spatula to prevent scorching.

Melt until smooth. Keep the heat low. As the mixture softens, scrape the bottom and sides so everything melts evenly. When it’s glossy and lump-free, remove from heat.

Add vanilla. Stir in the vanilla extract. If you like a balanced sweetness, add a small pinch of salt here to round out the flavors.

Customize (optional). Fold in 1/2–3/4 cup of your favorite mix-ins: chopped walnuts or pecans, mini marshmallows, or cookie bits. Don’t overmix or the fudge may lose its silky finish.

Spread and smooth. Scrape the mixture into the prepared pan. Use the spatula to press it into the corners and smooth the top. If using a topping like flaky sea salt or sprinkles, add it now.

Chill. Refrigerate for at least 2 hours, or until fully set. For a quicker set, place in the freezer for 30–40 minutes, but avoid freezing long-term before slicing to prevent condensation later.

Slice cleanly. Lift the fudge out using the parchment overhang. Warm a knife under hot water, dry it, then cut into neat squares. Wipe the knife between cuts for sharp edges.

Serve and store. Enjoy at cool room temperature for the creamiest bite. Store the rest as directed below.