Combine whole milk, heavy cream, cocoa powder, and granulated sugar in the slow cooker. Give it a good whisk.
Toss in the high-quality dark chocolate and give it a stir.
Cover and let it cook on high for 1 hour.
Uncover, stir until smooth and creamy.
Pour the hot chocolate into mugs, add marshmallows if you like, and savor the warmth!