Fully line an 8x8-inch square baking dish with aluminum foil or parchment paper so the bottom and sides are covered and excess hangs over the sides of the pan. Rub the butter all over the foil or parchment to help it peel off later.
Add the chocolate chips and condensed milk to a medium saucepan. Place over low heat and cook, stirring constantly, until the chocolate is melted and the mixture is smooth and thick, about 5 minutes.
Scoop the chocolate mixture into your prepared dish and smooth it with a spatula or the back of a spoon. Set aside to cool completely and set, about 3 hours, or chill for 2 hours.
When your fudge has set, remove the slab from the baking dish by peel lifting up the foil using the excess as handles. Peel off the foil and cut the fudge into squares with a sharp knife. You can enjoy the fudge immediately or wrap it in parchment or wax paper and store it in an airtight container for up to two weeks.
