In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Make a well in the center of the flour mixture. Pour in the milk, vegetable oil, and vanilla. Mix until combined.
For the topping, mix together the brown sugar, hot chocolate mix and cocoa powder, then sprinkle evenly over the cake mixture. Do not stir!
Finally, pour the hot water all over the top - do not stir!
Place the lid on the slow cooker and cook on low for 2 1/2 hours. Remove the lid and top with marshmallows and chocolate chips, then cook 30 more minutes. If the cake isn’t done, you can continue to cook it on low in 30-minute intervals until it is. There will be a gooey layer of pudding on the bottom, but the center should be a fully cooked cake.
Scoop out cake while warm and serve with your favorite non-dairy ice cream!