In a medium saucepan, warm 1 cup of heavy cream with the cocoa powder. Whisk until the cocoa is fully blended.

Bring the mixture to a boil. Remove from heat and add 3/4 cup of bittersweet or semi-sweet chocolate. Stir until smooth. Add the remaining cup of heavy cream and stir until smooth again. Pour the mixture into a large bowl, scraping the saucepan to remove all the chocolate. Set a mesh sieve over the bowl.

Warm the milk, sugar, and salt in the same saucepan.

In a separate bowl, whisk the yolks lightly. Slowly pour the warm milk, sugar, and salt mixture into the egg yolks while whisking constantly. Pour the mixture back into the saucepan. Stir constantly over medium heat with a heat-resistant spatula, scraping the sides and bottom. The mixture will begin to thicken and coat the spatula. It should also reach 170 degrees Fahrenheit / 77 degrees Celsius. Remove from the heat and pour through the mesh sieve into the chocolate mixture. Stir in the vanilla.

Place the bowl over an ice bath to cool. Transfer it to the refrigerator to chill thoroughly (approximately 25–30 minutes).

Pour the mixture into the ice cream maker according to the manufacturer’s instructions. Churn for 18–20 minutes. Add the marshmallows, walnuts, and chocolate chunks 2 minutes before the churning stops.

Store the mixture in an airtight, freezer-safe container. Freeze for 4–6 hours before serving.