Line an 8x8-inch (20x20 cm) pan with parchment paper, leaving an overhang on two sides to create a sling. Lightly spray the parchment with cooking spray if desired to prevent sticking.

Measure out the chocolate chips and sweetened condensed milk. Have a medium microwave-safe bowl (or a small heavy-bottomed saucepan), a rubber spatula, and your optional vanilla, salt, and toppings ready.

Add the chocolate chips and sweetened condensed milk to a medium microwave-safe bowl. If using the microwave method, do not add vanilla or salt yet; they will be stirred in after melting.

Microwave on 50–60% power for 30 seconds, then stir well. Continue to heat in 20–30 second bursts at 50–60% power, stirring after each burst, until the chocolate is mostly melted and the mixture looks thick and glossy. Stop while a few small lumps remain; they will melt as you stir.

Remove the bowl from the microwave and stir vigorously until the mixture is completely smooth and uniform, like thick brownie batter. Stir in the vanilla extract and fine sea salt, if using.

Alternatively, place the chocolate chips and sweetened condensed milk in a small heavy-bottomed saucepan. Warm over low heat, stirring constantly, until the chocolate is almost fully melted. Remove from the heat and continue stirring until smooth, then stir in vanilla and salt if using.

Scrape the warm fudge mixture into the prepared pan. Use a spatula to spread it evenly into the corners. Gently tap the pan on the counter to smooth the top and release any air bubbles.

While the fudge is still warm and soft, sprinkle chopped nuts, mini marshmallows, crushed peppermint, flaky sea salt, or sprinkles over the top. Press very lightly so they adhere without sinking too deeply.

Refrigerate the pan for at least 2 hours, or until the fudge is firm enough to slice. In a very full or warm refrigerator, this may take up to 3 hours.

Use the parchment overhang to lift the fudge slab out of the pan and place it on a cutting board. With a sharp knife, cut into 25–36 small squares, wiping the knife between cuts for clean edges. If the fudge is too hard straight from the fridge, let it sit at room temperature for 10–15 minutes before slicing.

Store fudge in an airtight container. At cool room temperature, it keeps for 2–3 days; in the refrigerator, up to 2 weeks. For longer storage, freeze for 2–3 months, well-wrapped or in a freezer-safe container, and thaw in the fridge or at room temperature before serving.