The quality of the chocolate used will affect the flavor and texture of the fudge. We prefer Ghirardelli semisweet and unsweetened chocolate in this recipe. Don\'t be tempted to make this fudge without the walnuts; they are crucial to the texture. If you prefer, you can use toasted nuts in this recipe. Make sure to remove the fudge from the double boiler before the chocolate is fully melted. If the chocolate stays in the double boiler too long, there is the possibility of the chocolate separating and producing a greasy fudge. This fudge will change texture and become drier the longer it is stored. Store the fudge, tightly wrapped in plastic, in a cool place for up to 2 weeks or in the freezer for 3 months. If frozen, allow ample time to let it reach room temperature before cutting.

1. Cut 12-inch length extra-wide heavy-duty aluminum foil; fold edges back to form 7 1/2-inch width. With folded sides facing down, fit foil securely into bottom and up sides of 8-inch-square baking pan, allowing excess to overhang pan sides. Spray foil with nonstick cooking spray.

2. Toss chocolates, baking soda, and salt in medium heatproof bowl until baking soda is evenly distributed. Stir in sweetened condensed milk and vanilla. Set bowl over 4-quart saucepan containing 2 cups simmering water. Stir with rubber spatula until chocolate is almost fully melted and few small pieces remain, 2 to 4 minutes.

3. Remove bowl from heat and continue to stir until chocolate is fully melted and mixture is smooth, about 2 minutes. Stir in walnuts. Transfer fudge to prepared pan and spread in even layer with spatula. Refrigerate until set, about 2 hours. Remove fudge from pan using foil and cut into squares.

Makes about 2 1/2 pounds