Line an 8×8-inch (20×20 cm) baking pan with parchment paper or foil, leaving overhang on two sides to lift the fudge out later. Lightly grease the parchment or foil to prevent sticking.

Add the chocolate chips and sweetened condensed milk to a medium microwave-safe bowl. If using, stir in the vanilla extract and a pinch of fine sea salt. The mixture will be thick at this stage.

Microwave on HIGH for 30 seconds, then stir very well. Continue microwaving in 20–30 second bursts, stirring after each, until the chocolate is about 75–80% melted (usually 60–90 seconds total).

Stop heating and stir until the mixture is completely smooth and glossy. The residual heat will finish melting any remaining chocolate chips. The mixture should be thick but pourable.

Scrape the fudge mixture into the prepared pan. Use a spatula to press it into the corners and smooth the top. If desired, sprinkle flaky sea salt, chopped nuts, or sprinkles over the surface and gently press them in so they adhere.

Refrigerate the pan for at least 2–3 hours, or until the fudge is firm enough to slice. For the cleanest cuts, chill 4 hours or overnight.

Use the parchment overhang to lift the fudge out of the pan and place it on a cutting board. Cut into about 36 small squares (or 25 larger pieces) with a sharp knife, wiping the blade with a warm, damp cloth between cuts for neat edges.

For the stovetop method, prepare the pan as above. Add the chocolate chips and sweetened condensed milk to a small, heavy-bottomed saucepan. Warm over low heat, stirring constantly, until melted and smooth. Remove from heat, stir in vanilla and salt if using, then spread into the prepared pan, add toppings if desired, and chill as directed.