Butter must be highly softened but NOT melted. I'd recommend leaving it out in a safe area for about 20 minutes.
With an electric mixer, cream the butter with 1 cup confectioners sugar and the milk.
Beat in the Nesquik and vanilla extract.
In small increments, beat in the rest of the confectioners' sugar. For a stiffer frosting, add a little more confectioners sugar.
This will ice about 24-36 cupcakes assuming you use about 2-3 Tbsp per cupcake. It's best to make this after you've baked and are ready to use it because refridgeration will cause it to set and make it practically unspreadable.