Combine sugar and buttermilk in a small Dutch oven, mixing well. Slowly bring to a boil, stirring constantly, until sugar dissolves. Cover and cook 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, stirring occasionally, until mixture reaches soft ball stage (234°). Stir in cream of tartar.
Remove from heat; stir in butter and vanilla. Beat 2 to 3 minutes or until mixture is creamy and begins to thicken. Stir in pecans. Working rapidly, drop mixture by rounded tablespoonfuls onto waxed paper; let cool. Remove from waxed paper, and store in an airtight container.