Heat sugar, butter and evaporated milk to full rolling boil in 3 quart heavy saucepan on medium heat, stirring constantly. Boil on medium heat until candy thermometer reaches 234F. (minus 2 degrees for every 1,000 Ft above see level), stirring constantly to prevent scorching, about 4 minutes. Remove from heat.
Stir in chocolate and marshmallow crème until melted, stir in walnuts and vanilla.
Spread immediately in foil-lined 9-inch square pan. Cool at room temperature at least 4 hours, cut into squares. Store in airtight container.