Prepare an 8 x 8 pan by lining it with parchment paper. Set aside.
In a saucepan over low-medium heat melt the butter, then stir in the chocolate chips and sweetened condensed milk until melted. Stir continuously. This should only take a few minutes.
You only want to melt everything together, DO NOT boil the mixture.
Pour the fudge mixture into the prepared pan, smoothing the top out with a spatula.
Cover and refrigerate for at least 2 hours, overnight is great!