In a large mixing bowl, use an electric mixer to whip heavy cream on medium-high speed until stiff peaks form, about 3 to 5 minutes.

Carefully fold in sweetened condensed milk into the whipped cream using a spatula.

Pour in chocolate syrup and gently stir it into the mixture until evenly distributed.

Crush Nutter Butter cookies and fold into the chocolate mixture; optionally add a swirl of peanut butter mixed with marshmallow fluff.

Pour the mixture into a freezer-safe container, cover tightly, and freeze for at least 4 hours, or until solid.