Line an 8x8-inch (20x20 cm) baking pan with parchment paper, leaving overhang on two sides to form handles. Lightly grease the parchment if you want ultra-clean edges.

In a large microwave-safe bowl, add the chocolate chips, the full can of sweetened condensed milk, and the pieces of butter. Do not add the vanilla yet.

Microwave the mixture on 50% power for 1 minute. Stir well, then microwave again in 20–30 second bursts at 50% power, stirring after each, until the chocolate is mostly melted and smooth. Avoid overheating.

When about 90% of the chocolate is melted, stop microwaving and stir until the remaining chocolate melts from the residual heat and the mixture is silky and smooth.

Stir in the vanilla extract and fine sea salt until fully combined. The mixture should be thick, glossy, and pourable, similar to brownie batter.

If using nuts, mini marshmallows, or crushed peppermint candy, gently fold them into the fudge mixture now. If using peanut butter for a swirl, wait to add it until after the fudge is in the pan.

Scrape the fudge mixture into the prepared pan. Use a spatula to spread it into an even layer and smooth the top.

If using peanut butter, spoon the warmed peanut butter over the surface and use a knife to swirl it through the fudge. Sprinkle flaky sea salt or extra crushed candy/nuts on top, pressing gently so they stick.

Refrigerate the pan for at least 2 hours, or until the fudge is firm in the center. For very clean squares, chill 3–4 hours or overnight.

Use the parchment handles to lift the fudge from the pan onto a cutting board. Cut into 1-inch squares with a sharp knife, wiping the blade with a warm, damp cloth between cuts. Let sit at room temperature for about 10 minutes before serving.