Spray the crock of a large slow cooker with cooking spray.
Whisk sugar, flour, cocoa powder, baking soda, baking powder, and salt together in a large bowl.
Whisk milk, oil, eggs, and vanilla together in a medium bowl until well combined. Slowly whisk in boiling water. Add wet ingredients to dry ingredients and mix until well combined; pour batter into the prepared slow cooker.
Cover and cook on Low until cake has no wet spots on top and is pulling away from the sides of the crock, about 3 hours. Turn off slow cooker and let rest for 30 minutes before serving.