Line an 8-inch square baking dish with HEAVY DUTY foil, allowing excess to overhang (see picture). Spray foil with nonstick cooking spray; set aside.
In a medium heat-proof bowl add chocolates, baking soda, and salt. Toss until baking soda is evenly distributed. Pour in sweetened condensed milk and vanilla; stir. Place bowl over a 4-quart saucepan with 2 cups of simmering water. Stir constantly until chocolate is almost completely melted and a few small pieces remain about 2-4 minutes.
Take the bowl off the heat and stir until chocolate is completely melted and smooth about 2 minutes. Fold in walnuts. Pour fudge into prepared pan and spread to make an even layer. Chill until set, about 2 hours. Use foil overhang to lift foil from pan and cut into squares.
This recipe makes 2.5 pounds of fudge. FOR A DOUBLE BATCH:
Line a 13 by 9-inch pan with two sheets of foil placed next to each other, overlapping slightly. This will allow the foil to overlap on all sides making handles.
Double all of the ingredients. In step 2, use a large heatproof bowl and Dutch oven containing 4 cups simmering water.
A double batch of this fudge will need about 3 hours in the refrigerator to set.