On medium to medium high heat, melt butter in a big, deep pot until mostly melted. Turn heat down to low. Add chocolate chips and sweetened condensed milk, stirring over low heat until creamy. Remove from heat as soon as creamy~ do not over cook. Add vanilla. Then, stir in walnuts and marshmallows. Pour into your dish of choice {fits perfectly in a 13x9}, lightly greased or parchment lined.