2 cups granulated sugar
1 cup buttermilk
1/4 pound butter
1 tablespoon light corn syrup
1 teaspoon baking soda
1 teaspoon vanilla extract
1 cup coarsely chopped walnuts

Using a 6-quart saucepan, butter upper sides of saucepan. Put all ingredients except vanilla extract and walnuts into saucepan. Grease a 10 x 5-inch pan.

Dissolve sugar, stirring constantly with a wooden spoon over low heat until butter melts and the spoon glides smoothly over the bottom of the pan. Increase heat to medium and bring to a boil. Wash down any crystals that may have formed with pastry brush dipped in hot water, using as little water as possible.

Clip a candy thermometer to the side of the saucepan. Reduce heat while retaining a boil, stirring no more than necessary. Cook until the mixture registers 236 degrees F to 242 degrees F, then shock the mixture by placing the saucepan in a sink filled with 1/2 inch of water. Add vanilla extract, without stirring, then allow the mixture to cool to lukewarm.

When mixture is lukewarm and a skin forms on top (110 degrees F), stir fudge thoroughly but not vigorously by hand, with an electric mixer, or with a food processor. Pause frequently to allow fudge to react. Watch for fudge to thicken, lose its sheen, become lighter in color or streaked with lighter shades, give off some heat, then suddenly stiffen. If mixing by hand, the fudge will "snap" with each stroke; by mixer, mixer waves will become very distinct; by food processor, fudge will flow sluggishly back to center when processor is stopped. Add walnuts before fudge totally candies. Pour, score and store when cool in an airtight container in the refrigerator or at room temperature. This yields 1 pound, but the recipe is easily doubled and can be frozen.

Variation
To slow-cook this fudge, combine sugar, buttermilk, butter, corn syrup and baking soda in a 6-quart saucepan and cook over very low heat for 1 hour. Stir occasionally to make sure it is not sticking to the bottom of the pan, then bring to a boil and proceed as usual. This results in a very, very creamy fudge.

Rum-Raisin Buttermilk Fudge
Omit the vanilla extract, but add, without stirring, 1 cup golden raisins soaked for 1 hour in 1/4 cup light rum and then drained, but not squeezed.