Make Peanut Butter Sauce: In medium heavy duty saucepan, stir cream, sugar, butter, and salt over medium heat to dissolve. Continue to cook, stirring frequently, until mixture comes to boil. Boil one minute.
Remove from heat and let cool until just warm. Stir in vanilla and peanut butter until smooth.
Make Hot Fudge: In medium saucepan, stir chocolate, corn syrup, water, sugar, cocoa powder, butter, and salt together over medium low heat until combined and melted. Increase heat to medium and bring to a low boil, stirring frequently. Continue to cook for 3 minutes, stirring frequently. Stir in vanilla off-heat. Allow to cool slightly.
Assemble sundaes: Set 4 to 5 whole peanut butter cups aside; chop remaining.
Layer 4 to 5 tall sundae glasses with ice cream, sauces, and chopped peanut butter cups. Top with whipped cream (if using) and a peanut butter cup.
