Combine butter, evaporated milk, sugar and salt in a 2 1/2 quart sauce pan. Cook and stir continuously over medium low heat. Bring to a boil while stirring constantly. Cook 4 minutes, while stirring vigorously (start timing when mixture starts to bubble around pan edge). Don't overcook. Remove from heat. Add marshmallows, vanilla, mint chips, and nuts (optional). Stir vigorously for 1 minute or until marshmallows and mint chips are completely melted and blended well. Pour into a 9 inch square buttered or greased pan (I like to line mine with foil and spray it). Allow to cool, then lift foil from pan and cut into squares. Store at room temperature. Enjoy!