Butter 8-inch square pan. Lightly butter side of 3-quart saucepan.
Combine buttermilk and baking soda in prepared saucepan, then add corn syrup, butter and sugar. Cook over medium heat, stirring constantly, until sugar dissolves and mixture comes to a boil. Wash down side of pan with pastry brush frequently dipped in hot water to remove sugar crystals.
Add candy thermometer; reduce heat to low. Stir mixture occasionally. Continue to cook until mixture reaches the soft-ball stage (238°F).
Pour into large heat-proof mixer bowl. Cool to lukewarm (about 110°F). Add vanilla and beat with heavy-duty electric mixer until thick. Beat in pecans when candy starts to lose its gloss. Spread in prepared pan. Score fudge into squares with knife. Refrigerate until firm. Cut into squares. Store in refrigerator.
Mixture boils up! Make sure to use a large (3-quart) saucepan.