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 and cocoa powder, then sprinkle evenly over the cake mixture.
Finally, pour the hot water all over the top - do not stir!
Cover the crock pot and cook on low for 3 hours. A toothpick should come cleanly out of the cake, but the bottom of the cake will have turned into a hot pudding-like mixture. If the cake isn’t done, you can continue to cook it on low in 30-minute intervals until it is.
Scoop out cake while warm with your favorite non-dairy ice cream!