Spray a 7 or 8 inch square pan with cooking spray. Line with parchment or wax paper with an overhang on two sides. (Good tip, crunch up the parchment paper before lining the pan.)In a small or medium size heavy saucepan, add the sugar, milk and butter. Bring to a boil. Stirring constantly. Reduce the heat to medium low and boil gently for about 3 to 4 minutes. Keep stirring often. Add the chips, stir until smooth. Now, and marshmallow along with the salt. Stir until nice and smooth. This may take a few minutes for the marshmallows to melt in. If needed add a 1/2 teaspoon of butter to make nice and creamy smooth. Take off the heat and add the vanilla extract. Stir once or twice until fully mixed in. Pour into the prepared pan and smooth out. Let cool on wire rack until completely cooled down. Then place in fridge for at least 2 to 3 hours. Cut into 20 squares. Nice to serve with fresh fruit.