In a mixing bowl, whip 1 cup of chilled heavy cream until stiff peaks form.

In another bowl, whisk together 1/2 cup sugar and 3 large egg yolks until smooth.

Pour in the melted dark chocolate and 1 teaspoon of vanilla extract into the yolk mixture, then gently fold in the whipped cream.

Repeat the previous steps with the second set of ingredients to create the vanilla layer.

In a freezer-safe container, alternate layers of chocolate and vanilla mixtures to create a striped pattern.

Freeze for at least 4 hours or until firm.