Place a 9×5-inch loaf pan in the freezer to chill.
To the bowl of a stand mixer, add the heavy cream. Start mixing at the lowest speed. Then, gradually increase speed to medium until the cream starts to get thicker. (The gradual progression prevents you from spraying yourself with cream.)
The cream will thicken and start to peak. Mix on high speed until the cream is thick and stiff. This happens quickly, so watch it closely! Set it aside.
In a medium, microwave-safe bowl, melt the chocolate chips in 30-second increments, stirring in between until creamy. Add sweetened condensed milk, cocoa powder, vanilla, salt, and 1 cup of whipped cream.
Gently fold the sweetened condensed milk mixture back into the mixing bowl with the rest of the whipped cream.
In a medium, microwave-safe bowl, melt the peanut butter in 30-second increments, stirring in between until creamy.
Pour half of the chocolate ice cream mixture into the chilled pan, drizzle on half of the peanut butter, top with the remaining ice cream, peanut butter, and then peanut butter cups. Swirl with a knife and freeze for at least 5 hours, or until fully set up, before serving.
Garnish with warmed peanut butter if using.