In a saucepan, place water, sugar, cream of tartar, and salt. Boil over high heat, without stirring, until candy thermometer reaches 248. Place marshmallow cream in a small bowl. Mix on low speed. Gradually add hot sugar mixture. Mix well. Fold in vanilla extract. Continue mixing on low speed until mixture begins to lose it gloss and hold it's shape when dropped from spoon. Fold in walnuts. Drop by teaspoonfuls onto wax paper. Cool and store in airtight container.