Add the condensed milk to a small saucepan. Pour the chips into the pan, give a stir, and make sure that the milk coats all the chips. Add a pinch of salt. Cook over low heat until it's melted and glossy and a looks like a thick, hot fudge.
Add the vanilla and walnuts, making sure to coat the nuts with the fudge. (I like to make double triple batches of this and freeze the extra.)
Line the bottom and sides of a 9-inch square baking pan with foil. You want to leave about 4 inches of overhang on the 2 opposite sides. These serve as your handles to remove the fudge from the pan in 1 piece, so make sure that there is enough of the overhang for you to have a solid grip.
Pour the fudge into the pan and smooth it out, getting it in all the corners. Let the fudge come to room temperature, then put in the fridge for 3 hours or so. This will make it easier to cut.