Step 1

Line an 8-inch square baking dish with aluminum foil, pushing the foil neatly into the corners and up the sides of the pan, using two pieces if necessary to ensure that the foil overlaps all edges (the overhang will help removal from the pan). Spray the foil with nonstick cooking spray.

Step 2

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

Step 3

Remove the bowl from the heat and continue to stir until the chocolate is fully melted and the mixture is smooth, about 2 minutes. Stir in the walnuts. Transfer the fudge to the prepared pan and spread into an even layer with the spatula. Refrigerate until set, about 2 hours. Remove the fudge from the pan using the foil overhang and cut into squares. Store the fudge, tightly wrapped in plastic, in a cool place for up to 2 weeks.

Step 4

TO MAKE A DOUBLE BATCH: Line a 13 by 9-inch pan; double amounts of all the ingredients; and use a large heatproof bowl and Dutch oven containing 4 cups of simmering water for melting the fudge mixture.

Step 5

Freezer-Friendly Instructions: The fudge can be frozen for up to 3 months. If frozen, allow ample time to let it reach room temperature before cutting. The fudge will change texture and become drier the longer it is frozen.