Whisk the 1/2 cup heavy cream, allulose, cocoa powder and butter in a small saucepan over medium heat. Simmer gently for 3 to 4 minutes, stirring often, until slightly thickened.
Remove the pan from the heat and stir in the chopped chocolate and salt until glossy. Let the fudge cool to room temperature, stirring occasionally.
Beat the cream cheese, powdered allulose, vanilla extract and salt with 1/4 cup of the cold heavy cream until smooth.
Whip the remaining heavy cream to stiff peaks and fold it into the cream cheese mixture until fully combined.
Gently fold in most of the mini peanut butter cups.
Spread one third of the ice cream into a loaf pan and drizzle with one third of the fudge. Repeat twice, then swirl lightly and top with the remaining peanut butter cups.
Cover and freeze for at least 6 hours. Let it rest for 10 minutes before scooping so the fudge ribbon stays soft and gooey.