Warm the milk and cream in a medium saucepan over medium heat, stirring frequently, until the mixture begins to steam. Add the sugar and stir until completely dissolved.
Meanwhile, warm the chocolate in a microwave for about 30 seconds to melt it. Add the chocolate to the saucepan and stir for about 3 minutes, until thoroughly combined. Whisk in the cocoa powder and salt until dissolved. Then reduce the heat and cook at a low temperature, stirring, for 2 to 3 minutes. Remove from the heat and stir in the Grand Marnier, if using.
Transfer the mixture to a 1-quart container, cover, and refrigerate for at least 6 hours. The chilled mixture will be quite thick, so give it a good whisk before pouring it into your ice cream maker. Freeze according to the manufacturer’s instructions. When it is finished, transfer the ice cream to an airtight container and freeze for at least 4 hours to harden.
Spicy Chocolate: Add 1/4 teaspoon (or 1/2 teaspoon for extra spicy) cayenne pepper and 1 cinnamon stick when you add the melted chocolate. Remove the cinnamon stick before chilling.
Smoky Chocolate: Add 1/2 teaspoon chipotle chili powder and 1 cinnamon stick when you add the melted chocolate. Remove the cinnamon stick before chilling.
Salty Chocolate: Add 2 teaspoons high-quality flaky sea salt or fleur de sel in a slow pour during the mix-in stage of your ice cream maker.