Line your baking dish with parchment paper, leaving some overhang for easy removal, and set it aside.
In a saucepan over low heat, melt the butter gently, listening for a soft sizzle and watching for it to shimmer, about 2 minutes.
Add the chocolate chips to the melted butter, stirring constantly with a spatula until the mixture is smooth, glossy, and fragrant, about 3 minutes.
Remove the saucepan from heat and pour in the sweetened condensed milk, stirring quickly until the mixture is velvety and well combined.
Pour the thick chocolate mixture into the prepared dish, spreading it evenly with a spatula and pressing down gently to create a smooth surface.
If desired, sprinkle chopped nuts or a pinch of sea salt evenly over the top for added texture and flavor contrast.
Place the dish in the refrigerator and chill for at least 3 hours, or until the fudge is firm and set.
Once chilled, lift the fudge out of the dish using the parchment overhang and transfer to a cutting board.
Slice into 12 evenly-sized squares with a sharp knife, wiping the blade clean between cuts for neat edges.
Serve immediately or store in an airtight container in the fridge for up to a week, bringing to room temperature before enjoying for a softer bite.
