Butter a 8” square baking pan. Combine buttermilk, sugar, butter, corn syrup and baking soda in pan. Stirring constantly, cook over medium-low heat until butter melts and sugar dissolves. Attach a candy thermometer to pan, making sure thermometer does not touch bottom of pan. Increase heat to medium and bring to a boil, Cooking without stirring until mixture reaches soft-ball stage (234-240º) . Remove from heat. Add vanilla; do not stir. Cool to approx 200º. Using an electric mixer on medium speed beat until fudge thickens and begins to lose its gloss. Stir in walnuts, Pour into prepared pan. Cool completely. Cut into 1” squares. Store in an airtight container in refrigerator.