Combine sugar, baking soda, salt, and buttermilk in a heavy saucepan; mix well. Bring to a boil. Stir in peanuts and butter. Cook over medium heat, stirring occasionally, until mixture reaches soft ball stage (240°).
Remove from heat; add vanilla, and beat with a wooden spoon 2 to 3 minutes or until mixture is creamy and begins to thicken. Working rapidly, drop by rounded tablespoonfuls onto greased waxed paper; let cool.