In a medium bowl, whisk together 4 large egg yolks and 1/4 cup of granulated sugar until well blended. Heat 2 cups of whole milk in a saucepan over medium heat until it reaches 165°F, stirring gently to avoid boiling. Gradually pour the hot milk into the egg mixture, whisking continuously to combine. Return the mixture back to the saucepan and cook on low heat until it thickens slightly, resembling a custard, for about 5 minutes.

Once your egg mixture has thickened, transfer it to a large mixing bowl and let it cool slightly. Stir in 2 cups of heavy cream, 1-2 tablespoons of vanilla extract, and a pinch of salt until everything is well incorporated.

Cover the bowl with plastic wrap and refrigerate the mixture for at least 4 hours, or overnight if possible.

While the mixture is chilling, chop 6-8 ounces of good quality dark chocolate into small pieces. Place the chopped chocolate in a heatproof bowl with 1-2 tablespoons of butter, then melt it over a double boiler or in the microwave on low power.

Once your base is chilled, pour it into an ice cream maker while it's running. Process according to the manufacturer’s instructions, usually around 25-30 minutes.

In the final minutes of churning, cut the tip of the piping bag to drizzle the melted chocolate into the thickening ice cream.

Scoop out the churned ice cream and transfer it to an airtight container, smoothing the top with a spatula.

Finally, place the container in the freezer for several hours, or until the ice cream is firm and ready to serve.