Fully line an 8×8-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.

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. Set aside to cool completely and set, about 3 hours, or chill for 2 hours.

When the fudge has set, remove the slab from the baking dish by lifting up the foil. Peel off the foil and cut the fudge into squares with a sharp knife.