In a microwave-safe bowl, combine the chocolate chips, butter, and sweetened condensed milk for the swirl. Microwave on high for 1 minute, stir well, then continue heating in 30-second bursts until the mixture is fully melted and silky smooth.

Mix in the vanilla extract, then chill the fudge mixture for 10 to 15 minutes until it is cooled but still soft enough to swirl easily.

In another bowl, whisk together the sweetened condensed milk, vanilla, and half-and-half until smooth.

In a separate bowl, beat the heavy cream with a hand mixer or stand mixer until stiff peaks form, about 1 to 1 1/2 minutes.

Gently fold the condensed milk mixture into the whipped cream until the ice cream base is smooth and even.

Transfer the ice cream base to an 8 x 8-inch pan or a similar freezer-safe container, then spread it into an even layer.

Take the chilled fudge mixture from the refrigerator and whisk briefly if needed to loosen any firmer spots. Spoon the fudge over the ice cream base in large dollops, distributing it across the surface.

Use a butter knife to swirl the fudge through the base, then freeze uncovered for 3 to 5 hours, or until firm.

For best texture, overnight freezing also works well. Cover leftovers with foil or plastic wrap and keep frozen for up to 3 weeks.