Butter the sides of a heavy 2-quart saucepan. Add sugar, sour cream, corn syrup and salt to the saucepan; cook over low heat, stirring constantly, until sugar is dissolved. Turn heat up to medium; cook until mixture boils. Continue cooking, stirring occasionally, to about 238° on candy thermometer, or soft ball* stage. Remove from heat.
Add butter and vanilla; cool to lukewarm or about 110°, then beat vigorously until fudge is creamy and no longer glossy. Stir in cherries and walnuts. Pour fudge mixture into a buttered 8-inch square pan. Let harden then cut into squares. Makes about 3 dozen pieces.
*To Test for Soft Ball Stage
A small amount of candy syrup dropped into chilled water forms a ball, but flattens a bit when picked up with fingers (234° to 240°).