First, prepare an 8 x 8 baking pan with parchment paper, so that the paper hangs over the edges of the pan. I used two sheets, so that the paper covered all the sides of the pan with a couple of inches extra all around. Set the pan aside.
Put the chocolate chips and the condensed milk in a heavy-bottomed saucepan or the top of a double boiler. Heat gently over a low flame, stirring, just until the chips melt into the milk—it will be very thick and the consistency should be uniform.
Turn off the heat. Stir in the vanilla and salt.
Scrape the fudge into the prepared pan and spread it evenly. Cover the top of the pan with foil, not touching the fudge, and put it in the fridge until it is mostly (ideally, not entirely) set, about two hours.
Lift the fudge out of the pan, using the parchment paper, and put it on a plate. Cut the fudge into cubes—whatever size you prefer. That's it. Store the fudge in the refrigerator.