Butter the bottom third of a 2 1/2 to 3 quart slow cooker.
In a medium size mixing bowl, stir together the flour, granulated sugar, 2 tablespoons of the cocoa powder, baking powder, and salt. Stir in milk, oil, and vanilla. Spread batter evenly in slow cooker.
In the same bowl you used for that mixture, combine the brown sugar and remaining 4 tablespoons cocoa. Stir in hot water until smooth. Pour evenly over batter in slow cooker. Cover and cook on high heat setting for about 2 1/2 hours.
When ready to serve, spoon ice cream into bowls, keeping all the ice cream cold as you serve each portion. With a big spoon, scoop up cake and sauce and set next to the ice cream