Line an 8-inch square baking dish with heavy-duty foil, allowing excess to overhang.
Spray foil with nonstick cooking spray.
MAKE FUDGE:
Add chocolates, baking soda, and salt in a medium heat-proof bowl.
Toss until baking soda is evenly distributed.
Pour in sweetened condensed milk and vanilla and stir.
Place the heat-proof bowl over a 4-quart saucepan with 2 cups of simmering water.
Stir constantly until the chocolate is almost completely melted and a few small pieces remain, about 2-4 minutes.
Take the bowl off the heat and stir until the chocolate is completely melted and smooth, about 2 minutes.
Fold in walnuts.
SET FUDGE:
Pour the fudge into the prepared pan and spread to make an even layer.
Chill until set, about 2 hours.
CUT FUDGE:
Use the foil overhang to lift the foil from the pan and cut it into squares.
Video
Notes
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 of simmering water.
A double batch of this fudge will need about 3 hours in the refrigerator to set.