In a bowl, whisk the sugar, flour, cocoa, baking soda, baking powder and salt.
In a separate bowl, whisk together the eggs, milk, oil, vanilla, and cinnamon until well combined. Whisk in the water. Pour the wet ingredients into the dry and mix well.
Pour the cake batter into the slow-cooker.
Set your slow-cooker on low.
Cook until cake has no wet spots on top and has pulled away from the sides of the crock, about three hours.
Turn off slow-cooker and let the cake rest for 20 minutes.