Heat 2 cups of milk in the microwave or on the stovetop. It just needs to be warm; don't boil it. Add the cocoa powder and whisk until combined (this helps the cocoa powder to blend really easily). Add this mixture to the slow cooker along with the remaining 4 cups of milk and all of the other ingredients.
Cook on high heat, whisking occasionally. It will take 30 minutes or so for the chocolate chips to melt, so be sure to whisk regularly until that happens. Once the chocolate chips are melted and the mixture is really warm, reduce the slow cooker to low or "keep warm".
Serving: Give the hot chocolate a whisk or stir before serving. Top each serving with marshmallows. Note: If you add the marshmallows directly into the slow cooker they will eventually melt.