1. Line an 8-inch or 9-inch square baking pan with parchment paper or aluminum foil, leaving an overhang on two opposite sides to form handles. Lightly grease the parchment to help the fudge release easily.

2. In a medium heavy-bottomed saucepan, combine the semi-sweet chocolate chips, sweetened condensed milk, and butter. Stir the mixture together before turning on the heat so everything starts to melt evenly.

3. Place the saucepan over low heat. Cook, stirring constantly, until the chocolate and butter are completely melted and the mixture is smooth and glossy. Do not let it boil or scorch.

4. Remove the saucepan from the heat. Stir in the vanilla extract and fine sea salt until fully incorporated. If using chopped nuts or other mix-ins, gently fold them into the warm fudge mixture.

5. Pour the fudge into the prepared pan, scraping the saucepan with a spatula to get every bit. Spread the mixture into an even layer and smooth the top. If desired, sprinkle extra nuts or a pinch of flaky salt over the surface.

6. Refrigerate the pan for at least 2 hours, or until the fudge is firm all the way through. For the cleanest cuts, let the fudge sit at room temperature for 5–10 minutes before slicing.

7. Lift the fudge from the pan using the parchment overhang and place it on a cutting board. Use a sharp knife to cut it into about 36 small squares or rectangles, wiping the knife between cuts.

8. Serve immediately, or store the fudge in an airtight container with parchment between layers at room temperature for up to 5 days, in the refrigerator for up to 2 weeks, or in the freezer for up to 2 months.