In a medium saucepan, whisk the brown sugar, cocoa powder and salt together until no lumps remain.
Slowly stream in the cream while whisking constantly.
Bring to a simmer, over medium heat, whisking occasionally, until the mixture comes to a simmer and the sugar is completely melted; about 3 minutes.
Remove from the heat; whisk in the chopped chocolate until smooth.
Add the peanut butter; whisk until smooth.
Let cool slightly before topping the sundaes if using immediatlely.
If not using right away, or for leftovers, pour into an airtight container and place in the refrigerator up to 1 week; to serve, re-heat it gently in the microwave or in a saucepan over low heat.
When ready to assemble a sundae, spoon a little bit of fudge into the bottom of a serving bowl or glass; top with ice cream, more warm fudge, whipped cream, chopped peanuts and a cherry.