In a large saucepan, combine 1 cup of heavy cream, whole milk, 3/4 cup granulated sugar, and 1/4 teaspoon salt. If using a vanilla bean, add the scraped seeds and the pod to the mixture. Heat over medium heat, stirring occasionally, until the sugar is dissolved and the mixture is hot but not boiling (around 175°F / 80°C).

In a separate medium bowl, whisk the egg yolks until light yellow. Slowly drizzle about half of the hot cream mixture into the egg yolks, whisking constantly to temper them.

Pour the tempered egg yolk mixture back into the saucepan with the remaining cream mixture. Continue to cook over medium-low heat, stirring constantly with a spatula, until the mixture thickens enough to coat the back of a spoon (about 175-180°F / 80-82°C). Do not boil.

Remove the saucepan from heat. If using a vanilla bean pod, remove it. Stir in the remaining 1 cup of cold heavy cream and 2 tsp vanilla extract (if not already added as vanilla bean). Strain the custard through a fine-mesh sieve into a clean bowl to remove any cooked egg bits or vanilla bean remnants. Place the bowl in an ice bath or refrigerate for at least 4 hours, or preferably overnight, until thoroughly chilled (below 40°F / 4°C).

While the base chills, prepare the chocolate swirl. In a small saucepan, whisk together cocoa powder, 1/2 cup granulated sugar, and 1/4 teaspoon salt. Gradually whisk in 1/2 cup whole milk until smooth.

Heat the chocolate mixture over medium-low heat, stirring constantly, until it comes to a gentle simmer and thickens slightly (about 3-5 minutes). Remove from heat, stir in the unsalted butter and 1 teaspoon vanilla extract until melted and smooth. Let it cool completely to room temperature, then refrigerate until chilled.

Once the vanilla ice cream base is fully chilled, pour it into your ice cream maker and churn according to the manufacturer's instructions, usually 20-25 minutes, until it reaches a soft-serve consistency.

In an airtight freezer-safe container, spoon about a third of the churned vanilla ice cream. Drizzle generously with about a quarter of the chilled chocolate swirl. Repeat layers, gently swirling with a knife or spoon after each chocolate addition, but do not fully mix. Finish with a final drizzle of chocolate on top.

Cover the container tightly and freeze for at least 4-6 hours, or until firm, before serving.