Mix crumbs, marshmallows, and walnuts in large bowl.
Place chocolate, milk, corn syrup, and salt in a 2 quart saucepan.
Cook over low heat until chocolate is melted.
Increase heat to medium, bring to a full boil, stirring constantly.
Boil approximately 10 minutes.
Remove from heat and add butter and vanilla; stir until well blended.
Immediately add chocolate mixture to crumb mixture.
Spread into greased 9 inch square pan.
Refirgerate for 3 hours or until set.
Cut into squares.