Grease a jelly roll pan or a rectangular baking pan with butter (do not use oil or oil spray).
In a large heavy bottomed pan over medium heat combine the butter, milk and sugar. Bring to a rolling boil over medium, stirring constantly with a strong, flat spatula.
Once it comes to a rolling boil, continue to stir over the same heat for five minutes.
Meanwhile put the marshmallows in a large microwave safe bowl and cook for two minutes or until they are pretty melted.
After five minutes, remove the milk mixture from the stove and add chocolate chips, stirring until melted. Add melted marshmallows, salt and vanilla – beat until well blended.
Poor into greased cookie sheet and cool on racks at room temperature. Cut into desired size squares when ready.
Store in an air tight container for short term or freeze for longer term storage.