Add the heavy whipping cream, sweetened condensed milk, whole milk, semi-sweet chocolate chips, vanilla extract, and cocoa powder into a 4-quart or larger slow cooker.
Whisk the ingredients vigorously to combine, ensuring the cocoa powder is mostly incorporated into the liquid.
Cover and cook on LOW for 2 hours. Stir occasionally (every 30-45 minutes) to ensure the chocolate chips melt evenly and do not stick to the bottom.
Once the mixture is hot and the chocolate is completely melted and smooth, switch the slow cooker to the WARM setting.
Whisk one last time before serving to froth the mixture slightly. Ladle into mugs and top with marshmallows or whipped cream.