Grease an 8" square baking dish with butter, oil or cooking spray. Line with waxed paper, leaving the edges of the waxed paper to hang over the edges of the dish.
In a medium, heavy bottomed saucepan, melt the chocolate chips with the milk over medium low heat. Stir in the butter until completely melted. Stir in the peanuts, vanilla and salt. Using a rubber spatula spread the mixture on the bottom of the prepared pan, smoothing the top. Cover loosely with plastic wrap and refrigerate until firm, at least 4 hours. Using a hot, dry knife, cut into 1" squares.