Line an 8x8-inch baking pan with foil or parchment paper and spray lightly with nonstick spray.
In a medium, heatproof bowl, toss the first two chocolates with baking soda and salt. Stir in the sweetened condensed milk and vanilla. Set the bowl over a 4-quart saucepan containing 2 cups simmering water. Stir with a rubber spatula until the chocolate is almost fully melted and few small pieces remain, about 2 to 4 minutes.
Remove the bowl from heat and continue to stir until the chocolate is fully melted and the mixture is smooth.
Stir in the marshmallows, peanuts, and additional chips. Scrape the fudge into the prepared pan and spread in an even layer with a rubber spatula.
Refrigerate until set, about 2 hours or longer. Remove the fudge from the pan using foil. Peel off the foil and cut into squares.
