In a heavy sauce pan combine milk, chocolate, vanilla, and salt over medium heat. Stir until melted and smooth. Remove from heat and fold in marshmallows and walnuts.
Line an 8x8 baking dish with plastic wrap and smooth out any wrinkles. Pour fudge into baking dish and smooth with rubber spatula. Top with mini chocolate chips. Refrigerate for 2 hours or overnight. Remove from pan, discard plastic wrap. Cut into squares, and store in an air tight container.