For the chocolate-peanut butter sauce: Warm the heavy cream in a small saucepan over medium heat until it begins to simmer. Place the chocolate chips and peanut butter in a medium bowl. Pour the warm cream over and allow to sit for 1 minute. Whisk until smooth. Set aside to cool slightly.
For the toppings: Using a hand-held mixer, whip the heavy cream and confectioners' sugar on high speed in a medium bowl until stiff peaks form. Using a rubber spatula, quickly fold in the marshmallow cream.
To make the sundaes, put 1 scoop of ice cream into each of 4 sundae bowls. Top each scoop with some of the Chocolate-Peanut Butter Sauce and a dollop of the Marshmallow Whipped Cream. Garnish with sprinkles, crushed cookies or crostini, as desired.