Start by lightly spraying the crock of your large slow cooker with cooking spray. This will help prevent the cake from sticking to the sides and make it easier to remove once it's finished cooking.
In a large bowl, whisk together the sugar, flour, cocoa powder, baking soda, baking powder, and salt until well combined.
In a medium bowl, whisk together the milk, oil, eggs, and vanilla extract until well combined. Slowly pour in the boiling water, whisking continuously until the mixture is well combined.
Pour the wet ingredients into the bowl with the dry ingredients and mix until everything is well combined. The batter should be smooth and free of any lumps.
Pour the batter into the prepared slow cooker and cover it with the lid. Cook on low heat for about 3 hours or until the cake has no wet spots on top and is starting to pull away from the sides of the crock.
Once the cake is finished cooking, turn off the slow cooker and let it rest for 30 minutes before serving. This will allow the cake to settle and make it easier to remove from the crock. Serve warm with a scoop of ice cream or a drizzle of chocolate sauce.
