Warm the milk, sugar, salt and ½ cup of the cream in a medium saucepan. Pour the remaining 1 cup cream into a large bowl and set a mesh strainer on top. In a separate medium bowl, whisk together the egg yolks. Slowly pour the warm mixture into the egg yolks, whisking constantly, then scrape the warmed egg yolks back into the saucepan.
Stir the mixture constantly over medium heat with a heatproof spatula, scraping the bottom as you stir, until the mixture thickens and coats the spatula. Pour the custard through the strainer and stir it into the cream to cool. Stir in the vanilla and stir until cool over an ice bath. Chill thoroughly in the refrigerator.
Freeze the ice cream in your ice cream maker according to the manufacturer's instructions. While the ice cream is freezing, chop the peanuts into bite-sized pieces.
As you remove the ice cream from the machine, fold in the peanut pieces layer generous spoonfuls of the sauce between layers of ice cream. Avoid stirring the fudge ripple, as it will make the ice cream muddy looking.
Put the pieces of chocolate in an absolutely dry heatproof bowl. Set the bowl over a saucepan of simmering water to melt the chocolate, stirring until smooth. In the meantime, stretch a piece of plastic wrap over a dinner plate.
Once the chocolate is melted, remove it from the heat and stir in the peanuts, coating them with the chocolate. Spread the mixture on the plastic-lined plate and chill.
Whisk together the sugar, corn syrup, water, and cocoa powder in a medium saucepan. Heat over medium heat, whisking constantly, until the mixture begins to bubble at the edges.
Continue to whisk until it just comes to a low boil. Cook for 1 minute, whisking frequently. Remove from the heat, stir in the vanilla, and let cool. Chill in the refrigerator before using.