Add the heavy cream, milk, cocoa, brown sugar, Kosher salt, and chocolate chips to the slow cooker. The cocoa will just float to the top at this point, but that's normal, the cocoa will mix in when you stir the chocolate chips later.
Cover and cook on low for 2 hours. Whisk every 30-45 minutes to keep the chocolate chips from burning to the bottom of the Crockpot as they melt.
Continue cooking until the hot chocolate is warm, steaming, and the chocolate chips have fully melted. Do not let the hot chocolate come to a boil.
Once the hot chocolate is smooth, stir in the vanilla at the end.
Serve right away, or switch the slow cooker to warm if you’re serving a crowd. Check the slow cooker every 30 minutes to make sure it's not boiling or forming a skin on top. If the chocolate starts to boil, turn it off and let it keep warm from its own heat.