Melt chocolate and 1/2 cup butter in top of a double boiler; stir in sugar, evaporated milk, and salt.
Bring mixture to boil; reduce heat and cook 4 minutes; stirring constantly.
Remove from heat; stir in vanilla. Let cool.
Combine chocolate wafer crumbs and melted butter, press firmly into a 13x9 inch pan. Cool.
Spread ice cream evenly over crumb mixture; top with chocolate sauce.
Spread with whipped cream.
Cover and freeze until firm.
Let stand at room temperature 5 minutes before slicing.