In a large mixing bowl, whisk together 1 3/4 cups of sugar, 2 cups of flour, 6 tablespoons of cocoa powder, 1 tablespoon of baking powder, and 1 teaspoon of salt.
Add milk, coconut oil, and vanilla to the dry mix, and stir.
Pour batter into a greased slow cooker and spread. It's normal for the batter to be a bit dry at this point.
In a medium bowl, stir together 1 1/4 cups of sugar and 1/2 cup cocoa powder, and sprinkle the cocoa sugar mixture on the batter in the slow cooker. DO NOT STIR. Pour boiling water over the cocoa mixture. Do not stir.
Cover and cook on high for 1 1/2-1 3/4 hours. You'll know it's done when the top cake part looks cooked, has pulled away from the edge, and a toothpick inserted comes out clean. The bottom is the hot fudge portion, so it should be smooth a pourable-looking.